Set up call forwarding, call waiting, or caller ID. (GSM) Go to Settings > Phone.

• Call Forwarding: The Call Forwarding icon () appears in the status bar when call forwarding is on. You must be in range of the cellular network when you set iPhone to forward calls, or calls won't be forwarded.
• Call Waiting: If you're on a call and call waiting is turned off, incoming calls go directly to voicemail.
• Caller ID: For FaceTime calls, your phone number is displayed even if caller ID is turned off.