If you were not quite fast enough to answer the phone before it stopped ringing, you can still find out the number that called. Almost all cellular phones offer a call history, which allows you to see the last calls made or received on your phone. If you have a land line phone, then dialing a special "Vertical Service Code" will give you the last number that called.
Press "Menu" on your cellular phone.
Select "Call History," "Call Records" or "Recent Calls."
Select "Missed Calls" or "Incoming Calls." The most recent missed call should be automatically highlighted.
Select "View" to see the number of the last call.
Land Line Phones
Pick up the receiver and wait until you hear a dial tone.
Listen for the recording relaying the last number that called you.