T-Mobile announced its readiness in November 2018. T-Mobile, the so-called un-carrier, is further differentiating itself from the crowd by being the first carrier to launch caller verification using the STIR and SHAKEN standards (no, James Bond was apparently not consulted). Customers that have been updated with the carrier's new technology will now see a "Caller Verified" label when a voice message has been confirmed as a real phone call and not one generated by scammers.
The STIR and SHAKEN standards allow calls to be signed as legitimate by originating carriers and validated by other operators before the calls reach consumers.
Caller Verified is an effort to fight caller ID spoofing, aka when scammers temporarily highjack a phone number that matches the area code and first three digits of the person they're calling, to make you think it could be someone you know. Then you'll have to wait, but perhaps not too long: T-Mobile said that "Caller Verified will be available on more smartphones later this year".
The service implements STIR (Secure Telephony Identity Revisited) and SHAKEN (Secure Handling of Asserted information using toKENs) standards, which the industry developed for this goal and has been promoted by the FCC for use right away.
Spam and scam calls are a big issue with mobile phones.
Once Verizon, AT&T, and Sprint adopt STIR/SHAKEN, Caller Verified will work across networks. They can be annoying when they ring your phone and interrupt you throughout your day, and they can be risky when they scam people out of money.