I think i have found the issue.
Sangoma has set this Values
This values dont exist, because it is a global value!
The right Value is
SIP_ADD_RPORT=Y (or N)
The only way was to let Panasonic TGP600 recognizing that the connection is lost, is setting this to "Y"
This Setting Refers to rfc3581
In my case!! I use TLS/SRTP and all Phones are external Phones.(NAT)
Snom has this Value enabled by default.
btw. the new Firmware. 06.001, which released a few weeks ago solves also a lot of issues with Randorm Ports. TGP600 sometimes used Random Port "0", which is resolved now with this firmware.