Hi:
This actually has nothing at all to do with whatever carrier you may be using.
It's a new scourge known as "neighbor spam": the robocallers load an entire local telephone exchange (e.g. 415-86x-xxxx) into their software, and then, "spoofing" other numbers in that exchange, call everyone at random. Their goal is to fool people into thinking it's a local business or neighbor trying to reach them about something legit, vs. seeing a far-away area code/prefix in the caller ID.
It's heinous, but there's not much you can do about it, other than to just not answer calls from caller IDs you don't recognize. You could also set up Google Voice with a "whitelist" of approved numbers, and then send every other call to voicemail, but it's a hassle.