Just a few comments from my recent experience as a Verizon voice customer:

Their global roaming with U.S. phones is now $1.29/minute in Western Europe and many other countries, and $2.49/minute max. I'm pretty sure that covers calls to both the local country and to the U.S.

You do need a GSM phone in Europe. (Not in many Asian countries though. A regular CDMA Verizon phone works fine. You just need to tell Verizon to turn on Global Service, no extra charge).

I bought a used Verizon GSM/CDMA phone on eBay, and Verizon gave me unlock codes they said would unlock the phone the first time I put a non-US chip in it. Haven't tried it yet, and don't know if they would do that with a new phone bought from them.