As far as my understanding, there is no public API available in iPhone SDK to find the users mobile number.
Can we find the mobile number of iPhone user using the Core telephony framework added in 4.0 SDK?
No, the CoreTelephony framework provides some basic information about the carrier and limited info about calls, not much else.
The limited information includes things like the amount of current active calls, the call ID (not a mobile number, just a unique ID that the OS uses to track the particular call) and the state of a call, like whether the call is on hold or not.
The main feature in CallTelephony is an API to check whether or not a carrier allows VoIP over 3G.