uaCSTA call receives BYE from the SIP phone T23G

[BrunoKonik]

Hello,

I have a SIP phone T23G 44.82.0.20.
An uaCSTA call is initiated from the server. The MonitorStart is correctly accepted. However after about 10 minutes, the server receives a BYE from the phone. I have tried to set the Keep Alive, to send an UPDATE from the server but none of this changes the problem.
What can explain the BYE sent ?

Thanks for your help.

Bruno