How would the know? They have been using California area codes to spoof numbers for years, just like they use 202 area codes for their IRS scams. They don't need to buy lists of numbers that would probably change daily. All they need is a simple computer program that dials every 4 digit combination in a given area code and exchange. Then they set it to whatever number and name they want to spoof.
I received a call from these scum bags at 9:00 this morning as I was sleeping, I did not answer, they left a message about lowering premium costs and no co-pay. I have insurance, and even if I didn't, i would not use someone that calls my home soliciting.