Here is a warning from an iPhone user:

BEFORE YOU BUY AN iPHONE, CONSIDER WHAT AT&T IS DOING:
AT&T SHUTS DOWN CALLS after 40% “off network use” . . .AT&T does not divulge what they are doing when you sign up or re-up. They flagged my account and I cannot use my iPhone when we travel where AT&T doesn’t have a tower for 2-3 weeks. They did not tell me this would happen because of this “flag.”
Individuals and Business people who travel for 1-2-3 weeks (depending on your billing period) cannot use an iPhone because of AT&T’s policy!