Extensions were still busy!

Dear Sir,

I have a strange problem in FreePBX Distro (2.10).

more than 400 phones are registered and work fine with the server.
BUT sometimes (one or two times per day), When a caller calls to a callee, and after that the caller end the speaking and pick down the phones, Asterisk does not send the HANGUP request!!. so with the command “sip show channelstats” I see two extensions that are talking about 23:10:50. (the connection does not HANGUP)
also, in this situation, Caller and Callee, both of them are still busy. and I had to send a command manually to HANGUP the connections. “channel request hangup sip/xxxx”

most of the phones are SNOM300 and YEALINK T22.

Could you please help me.
Thank you guys.

Have same issue, 100 extension with yealink t20, freepbx 2.11, at least one in month I need to do manuly soft hangup for extension remain inuse or active channel. RTP timers does not give results.