Asterisk (PBX)
SIP (Session Initiation Protocol)
H.323
IAX (Inter-Asterisk eXchange)
MGCP (Media Gateway Control Protocol)
Skinny (SCCP/Skinny Call Control Protocol)
UNISTIM (Nortel UNIStim/Unified Networks IP Stimulus)
XMPP (Extensible Messaging and Presence Protocol)