I use an ATT reseller called H2O (they also do Verizon) with an iPhone 3GS. Terrible customer service but most resellers have terrible customer service, luckily I don't need to call them often. You can pay by the month but I pay $100/year for 2000 minutes. You can even activate data at around 30 cents a MB which is way over priced but I don't use the phone much for voice so I turn on data to check eMail and such when I'm away from WiFi then turn it off again so the phone doesn't do a bunch of stuff in back ground and still have plenty of money in the account when I need to add another year (dollars roll over).