I don't see a reference to a local database of phone prefixes.
I did some research. The Phone app seems to be named Dialer on AOSP :
- The Android Telecom framework manages audio and video calls on an Android device.
- Screen calls. The spam check need network verification.
But too much information there.
It look more interesting here in the spam section :
An input to the discussion from someone more technical would be useful.
People looking at the source code use something better than the search field "Go to file" on Github ?
Don't know if it's a Google limitation but I can't use the search function in https://developer.android.com/