In the United States, the iPhone is sold exclusively for use on the AT&T wireless network. It is possible, however, to use a "jailbroken" and unlocked iPhone with T-Mobile's prepaid service, which offers both a prepaid unlimited calling plan and a pay-as-you-go prepaid service. As of 2010, you can purchase additional minutes for your prepaid T-Mobile service through the website or from on and offline merchants. You may check your prepaid T-Mobile balance directly from your iPhone at no charge.
Press the round button at the bottom of your iPhone's screen and slide your finger, as instructed, to unlock your iPhone.
Tap the telephone icon on your iPhone's screen and then press the keypad icon.
Press "#999#" and then tap the green telephone icon. Your current dollar and minute balance will appear on your iPhone's screen.
Log onto the T-Mobile website (see the Resources section), as an alternative to using your phone to check your number of minutes. The balance will appear on your account page.