I had this happen too back with GrandCentral, and it took forever to figure out because of the way it interacted with the PBX number it was forwarded to -- it w

I had this happen too back with GrandCentral, and it took forever to figure out because of the way it interacted with the PBX number it was forwarded to -- it would automatically hang up when you answered. I didn't figure out what was causing those calls until it became a Google Voice account, and saw the 0s calls in the logs (not sure if they showed up before). But GV gives you a major advantage over getting a new cell number with the same problem: you can block callers with one click.