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.