If your phone loses power or the power cord is unplugged from it during a conversation, you could see open channels in PBXware.

Solution for this is to check RTP Timeout option in  Settings->Protocols, SIP menu. Lower that value to 10 seconds which will tell the system to check RTP streams on the call every 10 seconds. If RTP stream is not present, which happens on phones power being cut, system will disconnect the call and free up that channel.