I have posted in that topic (https://helpdesk.snom.com/support/discussions/topics/6000046118), but there is no response there, so I post new one.
I have tried "disabling timer_support on the phone" and D710 no more adds Session-Expires: in SIP messages. But the problem is still present.
Furthermore with the settings: Timer Support (RFC4028):on when D710 send <INVITE> with Session-Expires: 3600 and Telesis response with 180 Ringing and 200 OK both without Session-Expires: the call is established normally.
So I attached trace from the phone itself without Session-Expires: Please answer in original topic and delete this one.
Please to avoid duplicate topics. We will continue in the other one. I have send you latest firmware in order to test with this.