It is indeed illegal under federal law to spoof identity and caller number as it arrives in caller ID displays, but there is something called "selective enforcement"
It seems that in the past 35 years, we've succeeded in "get government off our backs" - with this result, along with salmonella burgers, poisoned pet food, etc., etc.
Thus criminal telemarketer intrusion into our homes is on the "non-selected for enforcement" list...