In short and in plain language
An unknown number calls you. Often it is about selling or a scam. This post shows how to block such calls.
Your router, your phone and your mobile have their own blocks. You can reject calls that hide their number and block single numbers.
On your mobile you can silence unknown numbers. Your contacts still get through. So add your doctor, carers and family first.
A block has limits, because scammers can fake numbers. The post shows how to note down a call and report it.
We set up call blocking for you in your own home.
The phone rings, the display shows "Unknown" or a number nobody recognises. On the other end it is about a supposed prize draw, a cheaper electricity tariff or a story designed to frighten. If you receive calls like this often, you do not have to decide during every conversation whether you can trust the caller. Routers, cordless phones and smartphones come with blocking functions that stop a large share of these calls before they ring: rejecting anonymous calls, blocking individual numbers or whole number ranges, silencing unknown numbers. This article shows which block catches which type of call, where its limits are and how to record a call that still gets through so that the Bundesnetzagentur, the German Federal Network Agency, can work with it. It is written for everyone who answers the phone at home and for relatives who want to set this up for a father or mother.
Key takeaways
- 39,842 complaints about unlawful telephone advertising reached the Bundesnetzagentur in 2025, six per cent more than the year before (Bundesnetzagentur). Over 28 per cent of them concerned prize draws.
- Router and DECT phone block anonymous calls, individual numbers and whole number ranges. Under section 15 paragraph 1 TDDDG the provider must make rejecting anonymous calls possible free of charge.
- On a smartphone, unknown numbers can be silenced or sent to voicemail. Contacts still ring through, so the doctor's surgery, care service and family belong in the address book first.
- Blocking has limits: caller numbers can be spoofed (Bundesnetzagentur). Anyone calling from changing, genuine looking numbers slips through every block list.
- Advertising calls with a withheld number are prohibited under section 15 paragraph 2 TDDDG. For advertising calls without consent, section 20 paragraph 2 UWG provides for a fine of up to 300,000 euros.
- For a complaint, date, time, displayed number, company and advertised product matter. A screenshot of the call list helps; recording the call without consent is a criminal offence.
Why defence on the device comes before recognising scams
The figures from the Bundesnetzagentur show that unwanted calls are not a fringe issue. In 2025 the agency received 39,842 written complaints (Bundesnetzagentur) about unlawful telephone advertising, six per cent more than the year before with 37,561. Over 28 per cent of the complaints (Bundesnetzagentur) concerned prize draws; in 2024 it had been around 20 per cent. On top of that came 85,158 complaints (Bundesnetzagentur) about misuse of phone numbers; most of them concerned SMS and messenger spam and harassing call behaviour. Around 6,200 numbers (Bundesnetzagentur) were switched off by the agency in the same year.
How to recognise scams during a call is covered in the article Spotting phishing and fake calls and in the answer to the question How do I recognise fraudulent emails or calls?. This article is about the step before that. Every call that never rings is a conversation in which nobody can be put under pressure. Especially for people who live alone and politely see every conversation through to the end, this is the calmer solution: the technology filters first, and what gets through usually comes from known numbers. A feel for scams still matters, because no block catches everything.
What blocking cannot do
Which block catches which call
Not every function helps against every type of call. Before you change anything, look at the call list of the last few weeks: do the nuisance calls come without a number, repeatedly from the same number, from a particular range or from constantly changing numbers? That decides which block works. The table assigns the common cases to the devices where you can act.
| Type of call | On the router or DECT phone | On the smartphone | Limit |
|---|---|---|---|
| Call without a number, "anonymous" or "unknown" | Reject anonymous calls, alternatively ACR from the provider | Block private or unidentified numbers | Also catches calls that send no number for technical reasons |
| The same number calls repeatedly | Add the number to the block list | Block the number from the call list | Only works as long as the caller does not change the number |
| Many numbers with the same beginning | Block a number range, for example an area code | Varies by device and app | Also blocks wanted calls from that range |
| Constantly changing, unknown numbers | Only put known numbers through where the device offers an allow list | Silence unknown numbers or send them to voicemail | New contacts must be saved in advance |
| Spoofed, genuine looking number | No reliable defence | No reliable defence | Only your behaviour on the call helps here |
For most households a combination of two stages is enough: reject anonymous calls on the line and turn down unknown numbers on the mobile. If you are particularly badly affected, you can also work with an allow list. The Bundesnetzagentur describes this route for cases in which someone suffers very badly from harassing calls: you define the circle of numbers that may call, for example all saved contacts. All other calls are then no longer put through or are referred to a voicemail box.
Setting up call blocking on the router and DECT phone
On a landline there are two places for a block: the phone itself and the router it is connected to. The Bundesnetzagentur explains that on most phones the number to be blocked can be entered individually, or a certain group of numbers can be entered in full. This creates a block list of numbers whose calls no longer get through. If you use a router for telephony, you can make the setting there as well. The router is usually the better place, because the block then applies to all connected handsets at once and does not have to be maintained on each device separately. What this looks like on your device is shown on the page router setup.
For anonymous calls there is an additional route via the telephone provider. Under section 15 paragraph 1 TDDDG, called users must be able to reject calls with a withheld number in a simple way and free of charge. The function is called Anonymous Call Rejection, ACR for short (Bundesnetzagentur). Some providers let you switch it on in the customer portal, others do it on request. The advantage: the block happens directly at the provider, so the phones in the house do not have to be set up one by one. How to protect the router itself against access from outside is explained in the article Securing your home network and router.
- Open the router interface in the browser and log in with the password. The address is usually printed on the underside of the device or in the manual.
- Find the telephony section. Depending on the device, the blocking function is called something like call barring, block list or call blocking.
- Create a rule for incoming calls without a number so that anonymous calls are rejected, in case ACR is not switched on at the provider.
- Take individual nuisance numbers over from the call list instead of typing them in. That avoids typing errors.
- If needed, block a number range, such as a specific area code, if unwanted calls come from there in large numbers.
- Check with a test call: call the landline from a mobile with a withheld number and see whether the phone stays silent.
- Note down or photograph the settings so that they can be restored quickly after a device replacement or a change of provider.
Anonymous calls
Calls without a transmitted number are rejected, via ACR at the provider or via a rule in the router. Under section 15 paragraph 1 TDDDG rejecting them must be possible free of charge.
Individual number
A known nuisance number goes on the block list. This works against repeated calls from the same number and stops working as soon as the caller uses a new one.
Number range
All numbers with the same beginning are blocked, for example an area code. This reaches far, so check beforehand whether wanted calls also come from there.
Careful with calls without a number from important contacts
Setting up call protection on the smartphone
On a mobile the situation is different. It is less about anonymous calls and more about many unknown numbers. The Bundesnetzagentur advises not to call back straight away when you have missed a call from an unknown number. Both major systems offer settings that keep unknown numbers from ringing at all. The labels differ by version and manufacturer, but the principle is the same: whoever is in the address book rings through, everyone else lands quietly in the call list or on voicemail.
On an iPhone the route is Settings, Apps and Phone. Under "Unbekannte Nummern überprüfen" (screen unknown numbers), Apple offers the choice between "Nie" (never), "Nach Grund des Anrufs fragen" (ask the reason for the call) and "Stummschalten" (silence). Silencing makes sure that such calls are automatically silenced and forwarded to voicemail. With "ask the reason for the call", the caller is asked why they are calling before the phone even rings. Important for emergencies: after an emergency call, call screening is switched off for 24 hours (Apple Support). We practise the setup step by step in the iPhone courses.
On Android the route depends on the phone app. In the Phone app by Google, open the menu with the three dots, then Settings and Blocked numbers. There you can switch on "Unknown". Google's help describes exactly what this does: calls from private or unidentified numbers are blocked, but calls from numbers that are not in your contacts still come through. You block individual numbers in the call list via "Block/report spam". Some steps only work from Android 6.0 (Google). If visual voicemail is switched on, blocked callers cannot leave a message. Devices with their own manufacturer phone app name the menus differently; what this looks like on your device is what we show in the Android courses.
- Enter important numbers in the address book: GP, pharmacy, care service, neighbours, family, taxi.
- Silence unknown numbers or have the phone ask for the reason instead of blocking them completely.
- Block calls without a number, shown as "private" or "unknown".
- Switch on voicemail and check the greeting so that callers with a genuine concern can leave a message.
- Look through the call list regularly to see whether an important call slipped through silently.
- Block individual nuisance numbers directly from the call list.
Letting only contacts through means looking after the address book
Why withheld numbers are not allowed for advertising
Anyone calling with advertising may not hide their number. Section 15 paragraph 2 TDDDG prohibits withholding the caller number, or having it withheld by the provider, on calls made for the purpose of advertising. The Bundesnetzagentur puts it this way for consumers: advertising companies must display a number on advertising calls and may not call anonymously. If an advertising call arrives without a number or with a manipulated one, that is a separate administrative offence for which a fine of up to 300,000 euros (Bundesnetzagentur) can be imposed. With the TDDDG, this maximum fine rose on 1 December 2021 from the previous 10,000 to 300,000 euros (Bundesnetzagentur).
On calls made for the purpose of advertising, calling users may neither suppress the display of their number nor arrange for the provider of the telecommunications service to suppress it
For your defence this means two things. First, a household that rejects anonymous calls does not miss any lawful advertising call, because there is no such thing with a withheld number. Second, every advertising call without a number is an offence in itself that you can report. Regardless of that, a call for advertising purposes without your prior consent is unlawful (Bundesnetzagentur). For this, section 20 paragraph 2 UWG provides for a fine of up to 300,000 euros. Companies must also document consent and keep it for at least five years (section 20 UWG); a breach of this can be punished with up to 50,000 euros (section 20 UWG). According to the Bundesnetzagentur you can withdraw consent at any time, informally and without giving reasons.
The agency's record shows that these rules are enforced. In 2025 the Bundesnetzagentur handled 13 major fine proceedings (Bundesnetzagentur) and imposed fines totalling over 1.099 million euros. For the first time, breaches of the obligation to document and keep advertising consents were punished. In cases of number misuse, the agency prohibited invoicing unlawful charges or collecting such payments for more than 2,000 numbers (Bundesnetzagentur). Every complaint supplies the material for this.
Setup checklist to tick off
The following list covers a typical household with a landline and a smartphone. It can be worked through in one go, ideally together with the person who uses the phone every day. That way they know what changes and why a call no longer rings.
- Look through the call lists of landline and mobile: which calls are a nuisance, without a number, from one number or from many?
- Ask the telephone provider or check in the customer portal whether the rejection of anonymous calls (ACR) is switched on.
- Create a block for calls without a number in the router if ACR is not active at the provider.
- Transfer known nuisance numbers from the call list to the router's block list.
- Only block number ranges if no wanted calls are expected from there.
- Enter important contacts on the mobile before silencing unknown numbers.
- On the mobile, silence unknown numbers or have the phone ask for the reason, and block calls without a number.
- Check voicemail and answering machine so that callers with a genuine concern can leave a message.
- Use a test call with a withheld number to check that the block works.
- Tell the doctor's surgery, care service and family that anonymous calls no longer get through.
- Print the complaint template from the next section and place it next to the phone.
If you are setting this up for your parents
Recording a call: template for the complaint
If an advertising or scam call gets through despite the block, a complaint to the Bundesnetzagentur is the next step. For the agency to investigate, it needs details that are as precise as possible. In its protection tips it lists the caller's name, the product being advertised, the company or principal, date and time of the call and the caller's number. Suitable evidence is a screenshot of the call list or an extract from the router that shows the call came in. Write this down straight after hanging up, while your memory is fresh. The following template fits on a note next to the phone.
- Date and time: day, date and time of the call, as precisely as possible.
- Displayed number: exactly as it appears on the display or in the call list, even if it says "unknown".
- Line called: landline or mobile on which the call came in.
- Caller's name and company: how the person introduced themselves and on whose behalf they claimed to call.
- Content of the call: what it was about, such as a prize draw, electricity tariff, magazine or investment, and what was asked of you.
- Your response: whether you objected to further calls or withdrew an earlier consent, each with date, time and name of the person you spoke to.
- Evidence: screenshot of the call list or entry from the router's call list.
No recording without consent
The best way to submit the complaint is the complaint form on the Bundesnetzagentur's consumer pages. You can attach documents such as call lists, emails or contract papers that the advertising company sent you. For anonymous calls, the Bundesnetzagentur says a look at the router's call list can be worthwhile: some devices store the technically transmitted number of the line from which the call actually came. If there is such a note, it belongs in the complaint, because it is important evidence. There is one restriction: the agency can only impose fines for unlawful telephone advertising if the call was made to consumers.
Having call blocking set up during a home visit
Router and landline
Set up the rejection of anonymous calls, the block list and number ranges in the router, check the handsets and confirm everything with a test call.
Smartphone
Silence unknown numbers, enter important contacts, set up voicemail and show where the call list and the blocking function can be found.
Complaint template
Print the complaint template and place it next to the phone, and show how to take a screenshot of the call list.
The appointment takes place at your home, in the Hildesheim and Leine valley area. For the router there is the page router setup; if you would like to go calmly and step by step, Tech help for seniors offers the right setting. The home visit flat rate is 79 euros including VAT for an appointment of up to 60 minutes, travel within 10 km included; there is no subscription and no minimum term (prices as at September 2026). All prices are listed in the price overview, and for the city itself there is the page Tech help Hildesheim. You can arrange an appointment via Contact.
Sources
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