I’m using firmware 188.8.131.52 on UCM6102.
On 5 November all of our SIP trunks lost registration and refused to reconnect with no prior behaviour warning. All trunks were on the same provider.
I made no changes at that time, apart from setting an extension to forward unconditionally to an external number, which was done by logging in via VPN (VPN endpoint on main router, not UCM).
Each of the trunks could be connected from a softphone and there were no port blocks on the main router.
A trace showed that the initial registration was successful, but on subsequent renewals, the register request to the service provider was acknowledged with the security handshake correctly back to the UCM, but the UCM failed to send the authentication, and just responded with another register request.
A full configuration restore did not fix the problem.
However downgrading by one firmware, with a factory reset, re-upgrading and then restoration of the configuration has enabled the trunks to all register correctly again and to remain connected.
Grandstream are of the opinion that the call forward setting cannot affect the trunk registration. However I don’t have the time to delve into this at the moment, so I flag this up in case anyone else experiences a similar behaviour, with a potential fix, without actually finding out the cause.