Ralph,
I used to work for a LEC that didn't allow for "Spoofing" of numbers on calls made from their switch. If the phone system you were programming on was to be the source of outbound caller ID, and if the tech either mistyped or "Spoofed" a caller ID outside of the range purchased and made a call, the outbound Caller ID would be rejected by the switch and automatically insert the main telephone number on the account. If more LEC's did this I think there would be far less issues with TDoS, as the risk for getting caught is increased. Is my two cent on it anyways.
-Iron