... protocol: SIP(IMS compatible), seamless compatible with all popular call agent, including HUAWEI, ZTE, Alcatel-Lucent, etc. Integrate ... heartbeat function and support active/standby call agent.Voice coding: ITU-T G.711/G.723 ... redundant RFC2833, difference rings, MD5 authentication, call forward, call waiting, hot-line call, alarm clock, and all kinds of value-added ...