For several days, every incoming call forwarded through Google Voice appeared as Unknown. Testing showed that Google Voice displayed caller ID normally when the same account forwarded calls to other devices and carriers. That narrowed the problem to either the affected phone software or the carrier line.
I factory-reset the phone and installed another operating system, but the problem remained. T-Mobile support then tested the line and initially said replacing the number was unlikely to help.
What fixed it
Replacing the carrier number resolved the caller-ID problem immediately. In this case, the fault followed the original carrier line rather than Google Voice, the phone, or its operating system.
If the same symptom appears today, first test the Google Voice account with another destination number or carrier. That comparison helps separate an account-wide Google Voice problem from a problem tied to one forwarded line.
