Check out Core Telephony Framework: https://developer.apple.com/library/ios/documentation/NetworkingInternet/Reference/CoreTelephonyFrameworkReference/index.html
There is CTCall method, But CTCall identifying the call will only tell you the cureent state of call i.e is call being accepted or rejected. The callID of CTCall will just give you an unique identifier for that call but not the number being called.
NOTE your app should be in foreground state, otherwise it will not work.