VOIP SIP 2.0 ( RFC3261) and correlative RFCs.. Full duplex hands-free speaker phone. NAT transverse support STUN client. SIP support SIP domain, SIP authentication ( none, basic, MD5) , DNS name....
GX 50 Series Feature : 30 Concurrent calls Up to 100 registers Video Calls Multiple Language DID( Direct Inward Dialing Number) Support SKYPE for SIP Support USB disk recording( Scalable) ....