Go to Settings > Phone to:

  • See the phone number for your iPhone

  • Change the default text message replies for incoming calls

  • Turn call forwarding, call waiting, and caller ID on or off (GSM)

  • Turn TTY on or off

  • Change your voicemail password (GSM)

  • Require a PIN to unlock your SIM when you turn iPhone on (required by some carriers)

Go to Settings > Sounds to:

  • Set ringtones and volume

  • Set vibration options

  • Set the sound for new voicemail

Once you select a network, iPhone uses only that network. If the network is unavailable, “No service” appears on iPhone.