VOIP software uses voice over internet protocol and the voice signal is transmitted over the Internet or Packet Switched Network instead of PSTN(Public Switched Telephone Network).Voip software can be developed in many ways – i.e based on proprietary or open protocols.e.g IMS(IP Multimedia Subsystem),RTP(Real Time Protocol),SIP(Session Initiation Protocol)…and so on.Here is the two best and …