Incoming line drops immediately, sometimes

I have a very unusual situation that’s started a couple days back. We have two identical freepbx systems (don’t have the version handy but about 3 years old) both have Sangoma A200 cards. The 2 systems are tied together with an IAX trunk. All calls to the remote system are sent via the IAX to the main system where they feed into the time condition to either get sent to the reception ring group, or to after-hours greeting.

Now the problem… as of a couple of days ago “line4” on the remote system seems to have started picking up the call on about 1/2 ring (as heard by the caller) and then dropped after 9 seconds. The other two lines (2 and 3) are still working normally. If I look at the logs on both systems I can see that the transfer over the IAX to the main system proceeds but the call just drops before anyone can answer. Now the strangest part is that when the system switches to the after-hours greeting, that plays to the caller just fine (and it is also off of the main system). The only other oddity that I can see is that line4 isn’t showing any caller ID in the logs, whereas 2 and 3 do show caller ID… I wouldn’t think lack of caller ID should have any affect on who’s able to answer the line or not. I have been able to set up a temporary extension and voicemail box on the remote system and I can route “line4” into that mailbox… again behavior is kind of odd… the caller hears a half ring then about 8 or 9 seconds of silence before the voicemail greeting kicks in (the vm greeting kicks in at just about the same amount of time that the call would be getting dropped if I had it in it’s normal call-flow).

Have verified the config files on both systems and they are normal with no changes in over the past 2 years.

So the question is, has something funky gone wrong with port4 on the sangoma card… or has something gone wrong with the phone line… or both? Trying to think of any more remote troubleshooting that I can do until I can get hands-on with the system at the remote site.