Joerg > Caller ID must include the area code AFAIK.
Nope. Caller ID can be completely spoofed, both number and the text message.
A few special VOIP services specialize in this. Popular for Law Enforcement, skip tracers and private investigators.
Telemarketers don't even need that. Access to certain types of trunk level equipment also enable this, and are routinely used by telemarketers and teleservices companies.
There is a counterpart to caller ID used internal to telephone companies for billing and routing that is vastly more difficult to spoof.
You could theoretically run a counter-intelligence operation to trip up the people responsible.
Can you think of some really good bait to set up a sting?