I have seen that number on my Comcast Digital Voice detail of calls, and I feel quite sure that when I dial *99 to retrieve a voice mail message, (503) 908-0993 is the number the *99 takes me to.
503-908-0993 showed up on my Comcast Digital Voice detail of OUT GOING calls also. To the knowledge of anyone in my home, no one has dialed that number nor knows anything about it. I checked with Comcast and they could not give me any specific information on this number. When I dialed 503-908-0993 just now it came back as disconnected.