... queues.4) Integrated heartbeat function and support active/standby call agent.5) Support Voice Activity Detection (VAD)and Comfort noise Generation ... auto-negotiate with call agent.7) Echo cancellation exceeding ITU-T G.165/G.168-2002, up to 128ms tail ... reverse polarity, CID, MD5 authentication, abbreviation dial, call forward, call waiting, hot-line call, alarm clock, and all kinds of value-added ...