I am started getting reports of callers getting busy signals when placing inbound calls to TA924 Single PRI. Upon further checking the cause is due to 502 Bad Gateway. This is happening in spurts, comes and goes and callers can still place outbound calls while this is happening. I was able to check the PRI during one of these instances and it showed down, or at least that's what I think I saw because apparently outbound calls were still being placed during this time.
Look for errors on the T1 interface connected to the PRI. Check for timing slips in particular. Verify that the TA924 or the PBX is providing clock, not both and not neither. Also look at the PBX itself for errors.
Enabling B-channel restarts can sometimes help to keep things in sync if the PBX supports it.