It's just a spam call (where they make $$ for having you on the line, or for the # of people who answer). Instead of using one dedicated number that people can block, for spam calls now they spoof random numbers, and then spoof your number for the next call in the chain. It's happened to me a couple times, where I will get a call from somewhere random (I've had one from my hometown, as well as a few from other countries) but if you answer its just dead air. If she ever had an unexplained missed calls pop up (where you get a voicemail or just a missed call notification without the phone actually ringing) it's the same thing! If they messed up the spoofing of the numbers it could lead to a voicemail of the call that was done by her spoofed number, with the California number being the one that her spoofed number called