I know at least one person that has a software on his phone to spoof numbers...
I don't know if these softwares are available to public or only to cell phone company technicians (leaks?)...
But it does exist...
However, I don't know why clients would use spoofed numbers because when the agency or sp call them back, it won't be the same person...
It is always a good pratice I believe for SP or agency to call back for additional info or preferably SMS back... This will validate the number...
SMS is for me the best way to confirm the room # for incall. SMS insure back and forth messaging and confirm both numbers are valid.
Some agency/SP don't like to call clients back cause they don't want to create embarassing situation for the client... Well, these providers should know now that even if they don't answer any blocked call, for unblocked calls, some caller IDs are fake anyway (spoofed)...