And that replacable earplug thing rocks!
Ithis is only going to be a 4G phone, no LTE.
A 620? I don't see why you couldn't throw an AT&T SIM in it if you're so inclined. As long as it supports the right standards, GSM 850 & 1900 MHz and hopefully LTE too, it should work just fine. Just make sure you get an unlocked one. Bear in mind an unlocked one may cost more though. Even though I have 920, I'm digging the 620 too. Seems like it will be a great phone.