just got a call, missed it and called them back from the aller ID information.
"number does not exist" says the telephone company's automated system.
isn't Caller ID spoofing illegal?

Yes, for the most it is illegal.   However,  there are some measured exemptions.    Law enforcement,  Private detectives,  bail bondsmen,   and unfortunately debt collectors,    can use spoofing to a limited degree.   Just another hole in the government's  system of allowing certain considerations to its chosen few.

I missed a call from them and when I called back. They said it was for some home security system. So I hanged up since I already have some lol.

marketers, adt products...

Got a call from 657-202-9114 , Caller ID says PRIMEALM
Wasn't home to answer and they left no answer.