The number you see on your Caller ID is not necessarily the number from which the person called.
We can no longer believe the phone number we see on our caller IDs (CID) belongs to the person who called! The bad guys are now using easily accessed technology to "spoof" the CID numbers, so their real number does not appear.
The problem has gotten so bad, the US government actually passed an Act, called "The Truth in Caller ID Act of 2009", making such spoofing illegal. See http://www.fcc.gov/guides/caller-id-and-spoofing
for more information.