Originally Posted by dtsm

Safest model is Verizon as their iPhone from 4s model up are all factory gsm unlocked - can use both domestically and abroad (except sprint carrier).

ATT and/or TMO might require carrier unlock for international and domestic.

For LTE service, 5 or higher should work. You should be able to get quality 5s on craigslist....meet the buyer at a network store to make sure it's a clean phone.