If your iPhone has UMTS 850 & 1900 you might consider a
Straight Talk SIM. It's an "unlimited" monthly prepaid. You can have the SIM sent to your first US address with a month's service for about $45 if you use the promo code "SIMSAVE". They also sell an international calling bolt-on but you might want to just couple the Straight Talk account with a "dialthru" international service.