Mailbox-Trick

Four Methods to Identify Unknown Callers in Germany, 2026

Unsolicited calls from unknown numbers remain a common frustration, often leaving recipients to wonder: Is this a legitimate contact, a telemarketer, or a scam? As of May 2026, German media outlets have documented four practical methods to uncover the identity of unknown callers—without the risk of returning calls or revealing personal information. The most reliable of these, according to Merkur and Business Punk, is the Mailbox-Trick, which allows users to access a caller’s voicemail directly using provider-specific codes.

Reverse Phonebook Lookup: Limited but Possible

The first step for many is the reverse phonebook search, a method with clear limitations. Services like “Das Örtliche Telefonbuch” allow users to look up numbers, but only if the subscriber has opted into public directories. As reported by techbook.de and t-online.de, this approach works far more reliably for landlines than for mobile numbers, since the vast majority of German mobile subscribers do not list their numbers in public databases. For those with landline numbers, however, the method can provide quick answers.

Google Search: Quick and Often Effective

A more accessible method is a targeted Google search. By entering the unknown number in quotation marks, users can quickly find listings in databases like tellows.de or werruft.info, which often flag spam or scam numbers. This approach is particularly useful for identifying known telemarketing or fraudulent calls, as these numbers are frequently documented online with user warnings and descriptions.

Social Media Cross-Check: WhatsApp and Beyond

For those with social media accounts, a simple workaround exists: Save the unknown number to the phone’s address book and open it in WhatsApp. If the number is linked to a WhatsApp account, the user’s profile—including name and photo—will appear. As noted by t-online.de, this method extends to other messaging platforms that use phone numbers for verification, such as Telegram or Signal.

Mailbox-Trick: Direct Access to Voicemail

The most direct method, and the one most frequently recommended by German tech experts, is the Mailbox-Trick. By inserting a provider-specific code between the area code and the last digits of the number, users can access the caller’s voicemail without triggering a live call.

• Telekom: Insert “13” between the area code and number (e.g., 01511-13-1234567).
• Vodafone: Insert “50” (e.g., 01520-50-1234567).
• O2: Insert “33” (e.g., 01570-33-1234567).

This method works even if the number has been ported to a different provider, according to Merkur and Business Punk. Users should ensure their own number is blocked to avoid appearing in the caller’s call log. The trick is particularly useful for identifying business or personal contacts who may have changed numbers recently.

Datenschutz: Risks and Considerations

While these methods offer practical solutions, they also carry potential privacy risks. The WhatsApp cross-check, for example, relies on the linkage of phone numbers to social media profiles—a practice that exposes users to data visibility on both ends. The Mailbox-Trick, though not illegal in Germany, operates in a legal gray area, as it allows users to access information about others without their explicit consent. Experts advise against including sensitive information in voicemail greetings to mitigate the risk of identity theft.

For businesses and self-employed individuals, the methods underscore the importance of managing digital footprints carefully. A single unguarded voicemail or public directory listing can make personal or professional data more accessible than intended.

What’s Next for Caller Identification

As of May 2026, German regulators have not introduced new laws specifically targeting unknown caller identification, but ongoing debates about spam and fraud call volumes may lead to stricter measures. In the meantime, consumers are advised to use these methods judiciously, balancing convenience with privacy awareness.