Spoofed Spam phone calls

Is there anyway to prevent the spammers who spoof their numbers from calling your cell phone?

Seems Verizon must have sold/made available my number (now an AT&T customer for sometime, but the number was originally a Verizon one).

Of course when I try the number back...it's always a number not in service==spoofed number.

I've tried the do not call registry..but that doesn't seem to work...after a week...they are still coming in.