Port causes congestion

I have a pbx using sangoma cards. We have 6 ports on the cards and 5 ISDN lines fed into these, leaving one spare. I had a big issue with congestion that is now solved, but I was wondering if someone could help clarify what they believe the issue was.

Basically if I plug into port one, I can make and receive calls not a problem. Same goes for the other 6 ports and the cable used does not make a difference.

However even with all 5 cables in the outgoing limit is 3 when a cable is plugged into port one. I changed the cable form port 1 to port 6 and the issue was resolved. The pbx has been working absolutely fine until a couple of days ago. This was not due to any change on the system whatsoever. But basically whenever a cable is plugged into port 1 we have a call concurrent outbound call limitation.

What would cause this? Like I say it has been working fine, a single cable into port one allows incoming and outgoing calls suggesting no issue with the port, however it causes congestion when in use. I’m really just curious at to why this would have happened.

Thank you for taking the time to read this and any thoughts are welcome.

Probably a hardware problem, but check with Sangoma first. . .