The Apple iPhone is currently restricted to AT&T's cell network due to an exclusive contract between the two companies. However, there are ways around this restriction. Installing modifications that allow an iPhone to work on other networks is known as "unlocking" an iPhone. There is no immediate way to tell if an iPhone is unlocked or not, especially if purchased from a secondhand source. However, with the use of another cell carrier's SIM card, the unlock status of a iPhone can quickly be determined.
Power down the iPhone if it is on. Locate the SIM-card cover on the iPhone. The SIM-card cover is located between the headphone jack and the sleep button. Insert an unfolded paper clip into the narrow hole next to the slot to open it.
Remove the standard SIM card from the slot. Insert the T-Mobile SIM card into the slot and close the cover. T-Mobile uses the same GSM band used by AT&T, which will allow the iPhone to function if it is unlocked. SIM cards from other carriers will not be functional in an iPhone regardless of the unlock status.
Power up the iPhone and wait two minutes to allow it to adapt to the new card. Place a call to another telephone number. If the call connects, the iPhone is unlocked. If the call fails to connect, the iPhone is not unlocked.