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The companies are now verifying whether calls between their networks are authentic - in other words, that the caller is a human.
It doesn't stop robocalls, but at least it makes it easier to spot and ignore them.
Think of each phone line as having a security pass. Comcast and T-Mobile are checking that ID before passing the call onto you, and good ones will show up as "caller verified" on your screen.
It's early days for this tech: it'll work first on 10 of T-Mobile's Android devices and only for calls across the two networks.
Comcast and T-Mobile are working with other providers to get call verification up and running on more phones and networks.