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.