If all else fails that you are all working on; problem could be with the carrier's CO and they may have their routing incorrect for that particular number.
This is not uncommon to happen. Your home location could be processing the call thru CO A, B, and C. Any of the CO's could have their routing for that number screwed up.
When you make you call thru the ties and go out via another pbx, that pbx may have the call processed through CO's B and C. ..thus completes because CO A has routing messed up.
Making a test call via a cell phone also will prove nothing new other than it is via number. Because cell phone calls are processed differently, mainly through cell towers, then to final destination CO.
So if all else fails, escalate to the carrier.