sg-920 is a kind of voice communication terminal which is based on wide band and independent of pc (personal computer). voice features  support sip rfc3261  support iax2  codec:g.711a/u, g.7231 high/low, g.729  echo cancellation:support g.168  support voice gain setting, jitter buffer, vad, cng  nat transverse: support stun client, citron, avs  sip support sip domain, sip authentication (none, basic, md5), dns name of server, peer to peer  sip support pubic/private server. can connect to public isp and private sip server at the same time  sip support dual public server  dtmf:support sip info, dtmf relay, rfc2833  sip application: support call forward/transfer/holding/waiting  call control features: flexible dial map, support hotline, empty calling no. ....