The area code and prefix of the monitor number identified which central office the wiretap was in. If Lewis or I knew anyone who had a phone number served out of a CO where a wiretap was active, I would call the central office, say I was from PacBell Security, and explain, "We have one of our boxes there. I need you to trace out the connection." After a couple of steps I would have the target phone number that the intercept was placed on. If it didn’t belong to anybody I knew, I’d go on to explore the next one.