Answer
You enable Silent Operation Mode by setting S504 to 1. Setting to 1 forces S0 to -1 and suppresses messages arising from connections or pairing. (ie. CONNECT, NO CARRIER, RING, PAIR etc.). Suppressing connection based messaged allows the device to configure in cable replacement mode. It cannot be enabled/disabled on the fly. You would need to enter "AT&W" and "ATZ".