GNU SIP Witch is a secure peer-to-peer VoIP server. Calls can be made even
behind NAT firewalls, and without needing a service provider. SIP Witch can
be used on the desktop to create bottom-up secure calling networks as a
free software alternative to Skype. SIP Witch can also be used as a stand-
alone SIP-based office telephone server, or to create secure VoIP networks
for an existing IP-PBX such as Asterisk, FreeSWITCH, or Yate.