[Splines]

iOS/Android could do something like this. You register your number with Apple/Google and link your account with them. When you call someone you set a field on your account that you're calling someone. When the person who you're dialing gets rung, their dialer can look up Apple/Google and see if that number was indeed calling them, and add a "verified" checkmark to the call.

This leads down a privacy/metadata rabbit hole, but there are probably ways to make this a lot better. In any case, the phone OS can do some out-of-band signaling and just avoid dealing with the carriers altogether.

Although if you're doing all that then why not just make a call using voip...