It should not be too difficult. Having spent over 30 years running my own telecoms company I can tell you that there are two CLI's (Calling Line Identities) this is the number that identifies the source of the call.
The Presentation CLI is what appears on your phone, or does not if you have not paid for it, if the call is from an Ex Directory Number, or if the caller has witheld it. This number can be the number of the line the call was made from or can be another number associated with the "owner" of the line. For example XYZ Company may have Direct Dial Numbers for all of its extensions but might present the Sales Line number if the call has come from a salesman or the main switchboard number if it was the MD calling.
The other CLI is the Network CLI, which actually identifies the source of the call. This is kept within the network and does not reach the called party, it does however form part of the carrier's call record and so should be relatively easy to locate. Even if the caller witheld his number the network CLI is still there.
There will be a very few calls where the data is not available but these usually involve calls from places where they are still using fairly ancient exchange equipment, typically overseas.
Of course if it's a pay as you go mobile bought over the counter in a supermarket it can be more tricky tying the originating number to an individual.
Last edited by Mike Cross; 4th Oct 2013 at 18:03.