"Terminator Blocked" on MD110

[voipuser]

Let me Preface by saying I'm not very familiar with PABX programming. I've had some contact with PBX's but not much. Setup is as follows on the far end there is the customers MD110 (not sure of software revision) that interfaces via analog trunks with our equipment an anolog to digital channel bank which then goes t1 to a cisco router. The issue is that our customer from time to time gets a "Terminator Blocked" condition on the PBX after entering an access code. I don't know why, everything on our side is fine. I can hook up a buttset to the punch down and complete a call normally. I suspect this is an issue that a newer revision of software can resolve. Has anyone seen this in the field? Is it a bug in old software? Do we have a signaling issue? My channel bank can do either loop or ground, I'm not sure what the PBX can do. Oh, also if he send a ringback test tone via the channel bank it clears the line on the PBX. Their PBX tech doesn't know what the problem is. Any suggestion are greatly appreciated.