To display a phone number as the caller ID for outgoing voice calls, you have to verify it once.
Add a phone number
Open Settings -> Sender IDs, click New Sender ID and enter the phone number you want to verify. Save the entry.
The new number appears in the list with a Verify button.
Start the verification
Click Verify. The Verify Caller ID dialog shows how many verifications you still have left this week (max. 2 per number). Make sure the phone is at hand and ready to receive calls.
Click Start Verification to trigger an immediate call to the number. A voice reads out a 6-digit code which you type into the dialog's input boxes.
Once the code is entered correctly, the number is enabled as a voice sender.
Caller ID display on international calls
The verified number is passed on as the caller ID with every call. For calls to numbers outside Germany, however, the carrier in the destination country decides whether it forwards a foreign caller ID. If it rejects the caller ID, the recipient sees a seven number or a withheld number instead. A guaranteed display of the verified number abroad is therefore not possible. Test the display with a call to the destination country before using it in production and contact support@seven.io if it differs.