That was the first step. In order to hear audio on the line—voices, noise, static, or whatever—the tech would then have to establish an audio connection to the SAS unit in the CO. These units were designed with a clever security provision: they had a list of phone numbers preprogrammed into their memories. The technician would have to send a command to the SAS unit to dial back to one of the preprogrammed numbers—the phone number at the location where he was working.