Ok, here's what we got.

• A Snom M300 plus two M25 headsets
• A Lancom router 1790VA playing the PBX with the current firmware.
• A Gira TKS-IP-Gateway with the current firmware.

Now when I recently upgraded the firmware of the M300 from 530B2 to 530B7 the base station started crashing completely and reproducible as soon as you answered a call from the door station. The M300's log says afterwards

Unexpected Reboot (50) Firmwareversion 0530.0007 (RESET_CAUSE_HARDWARE_RESET)

Now the odd thing is that this previously worked perfectly fine with the same firmware versions on the Lancom and the TKS-IP-Gateway and 530B2 as the the DECT system's firmware.

So I downgraded the base station and the headsets to 530B2 again. And everything is fine again.

What has changed for the worse in 530B7?