I have a GrandStream ATA device which reports it is correctly registered. However, my Freshphone dashboard says my device is Offline. When I reboot my GrandStream, it intermittently goes Online briefly before going Offline again. Needless to say, I can't make or receive calls.
Issue solved by enabling [NAT Traversal = 'Keep-Alive'] in the GrandStream. This was not required before, but a fibre configuration change probably made this a new requirement.