Got a nasty call from a telemarketer? Or even a completely and blatantly fraudulent call? Got the number from Caller-ID? Wanna call them back and blow a foghorn, or even report the number to someone?
Spoofing of telephone numbers in these situations is so common as to be almost mandatory.
It's not even hard.
First off, even with a regular phone call, the number you see with Caller-ID is transmitted via a normal Bell 212A modem signal between the first and second rings of the phone. If you've still got a modem around, you can do this yourself, and spoof a specific number, or at least flood the buffer and "flush out" the real one.
Secondly, most "call shops" these days are going to be using some kind of Voice over IP system. Originally, these might have defaulted to the number of the interface from the data network to the phone network, but these days it's pretty much user settable.
Telephone number spoofing has become common practice even for so-called legitimate companies.
I own a timeshare vacation week. (contact me directly for comments on time shares). The timeshare resort where I own is owned and managed by Diamond Resorts* of Las Vegas NV. I have received repeated calls from Diamond Resorts with phone numbers spoofed to appear to be coming from in or near my Virginia community. Back before I got NoMoRobo** I answered some of those calls. The callers, who said they were variously in Nevada, Florida, or California, freely admitted using the spoofing.
*Search the TV show Undercover Boss for two appearances by the former Diamond Resorts President and owner. He sold the company last year.
** I highly recommend this service.