HI Alejandro. Would be interesting to see snapshots of the Status>ISDN Page on the ISDN GW if possible. If/When Layer 1, 2 or both is down on the GW, like you stated it would deregister from GK until the system is back online. You can send your messages to syslog server and look for entries from the Port Manager module indicating Layer 2 is down or Layer 1 physical is down etc.
So if you look at the Status>ISDN page on GW and says L1 and L2 are up and then navigate to ISDN>ISDN Ports and it says the same, would be interesting to see the event log from the GW while its in this state so we can see the events that happened prior to this to try and ascertain what is going on.
I know this doesn't help you now, but would need more intel to make an assumption here.