secure peer-to-peer VoIP server for the SIP protocol 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.