A smartphone with an incoming phone call from an unknown caller.

Enlarge (credit: Getty Images | Tero Vesalainen)

AT&T and T-Mobile announced a joint anti-robocall initiative today, but they didn't promise any new call-blocking capabilities for their customers.

The carriers made a big deal of the partnership, saying in an announcement that they "put differences aside to fight unwanted robocalls for customers." Specifically, the companies said they are now using the new SHAKEN/STIR technology to determine whether Caller ID numbers are being spoofed in calls made between the two carriers.

Theoretically, carriers could use this Caller ID authentication technology to automatically block calls that fail the authentication test. But that's not what's happening now. For example, AT&T told Ars that it's using Caller ID authentication as one data point in its anti-robocall algorithm but that it isn't blocking calls solely based on whether they aren't authenticated.

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