Unlocked ATT iPhone not working with verizon sim card?

Only the internet and messages work but calls do not work

Verizon is a CDMA carrier so the phone would have to be activated/flashed to them in order for it to work in terms of its phone functions.

That's because Verizon requires a special version of the iPhone. Verizon uses the old CDMA system, instead of the modern GSM system used by just about everyone else in the world (including AT&T). Your iPhone is a standard iPhone, which will work with most of the world's carriers, but not Verizon. The SIM card for Verizon is used only for roaming and non-CDMA functions, but calling and texting require CDMA. You'll have to get a new Verizon iPhone if you really want to stay with Verizon.