Can you buy a Iphone 4 and buy a sim card that is on a pre-paid plan?

Like cricket, virgin mobile or any kind of pre-paid plan you don't have to buy your phone from that company.

Straight Talk prepaid and a few other smaller services out there let you do this. But the benefit of ST is that the speed is comparable to contract plans from AT&T and T-Mobile (because the service runs off these companies' cellular equipment) and the phone you buy does not need to be unlocked because ST sim cards are designed to trick the phone into thinking that the sim card is of the carrier lock.

Yes. You can buy an unlocked iPhone or a jailbroken iPhone then buy a sim card for a prepaid plan separately. I can confirm that one carrier -- Simple Mobile -- does not require an unlocked phone to use their service.

If you use a prepaid service with a provider other than the official providers you lose built-in visual voicemail. But there are several apps that you can download that link to free services that give visual voicemail and expanded greeting options as a bonus. You will also need to change the APN setting on your phone to get data and MMS. There are instruction for this online and you can find a link to Howard Forums below.

I found several prepaid services that can use a GSM iPhone. I did not check CDMA iPhone services as I don't currently own one. Cricket and Virgin Mobile are CDMA carriers. The breakdown of the best alternate options I found was:

1) Straight Talk $45 Unlimited plan (plus tax)
• The coverage map is the same as AT&T -- they use the ATT wireless network infrastructure
• You DO NOT need to jailbreak the phone to get basic service.
• The iPhone 4 is 3G capable you will get full 3G speeds on this service.
• Auto renew available to ensure continuous service at the end of each 30 day prepaid contract.

2) T-Mobile
• iPhone not officially supported, it will work -- but they won't help with problems.
• $50 Unlimited plan (plus tax) Unlimited Phone, Text and data BUT the iPhone will ONLY GET 2G.
• $30 1500 min or texts + Unlimited data plan (plus tax) -- again at 2G

3) Simple Mobile $40 (plus tax) -- they use the T-Mobile wireless network infrastructure
• iPhone not officially supported, it will work -- but they won't help with problems.
• Same coverage map as T-Mobile
• No option for auto-renew on site. Must call in or renew online.