IPhone Flash Drive

• Unlike some cellphones, which store data on SIM cards or removable SD cards -- as is the case with digital cameras -- the iPhone has a "flash" drive inside it, which functions similarly to your computer's hard drive. The iPhone's SIM card serves only to assign the iPhone a carrier and a phone number. Optionally, it's possible to copy contacts stored on a SIM card onto an iPhone, but this is a one-way procedure that doesn't work in reverse.
  
  

Reading Messages on iPhone

• Messages received on an iPhone remain on the iPhone until you delete them, regardless of how many SIM cards you use in the device. As a result, reading a message stored on an iPhone requires only that you click the "Messages" icon and scroll through the list of messages until you see the one you wish to read.
  
  

Replying to Messages

• The primary problem that arises using different SIM cards in an iPhone is not reading text messages but rather replying to them. The SIM card assigns a phone number to an iPhone, so when you reply to a message you have stored on the device, the recipient sees it in conjunction with the phone number assigned to the SIM card you're using, regardless of the number to which the person originally sent it. Include an addendum in a message you send from a different SIM card to avoid confusing the recipient.
  
  

Non-Supported SIM Cards

• Officially, only AT&T supports SIM-bearing iPhones in the U.S. -- inserting a non-AT&T SIM into an unmodified iPhone produces an error message. It is nonetheless possible to read messages on such a device, but unless you have "jailbroken" the device -- an unauthorized procedure AT&T and Apple strongly discourage that allows the device to be used with any carrier -- you aren't able to reply to a text message stored on the iPhone for any reason.