This was at a previous job, more related to systems administration then programming, and not my mistake but I figured out how to fix it (ok, maybe not on topic, but close enough haha)
We were opening a new office in a different state and the PBX was to connect back to the home office over an MPLS.
I and the phone techs flew down there, got the network up and running, all the equipment installed, everything else. I got back to the home office to be there for the final install and nothing. We could not call from the home office to the remote office; but we could call from the remote office to home.
2 days and nights, 8 of us in the server room at home, a few more in the remote office - checking cables, checking router configs, trying different hardware thinking it was hardware failure, on the phone with the ISP / PBX provider.
Finally, exhausted - it hit me "Wait wait wait. What did we learn going through PCI compliance? Never leave un-needed ports open. Has anyone actually checked if port xxxx is open on the PBX to communicate with the remote office?"
5 minutes later, calls were going through just fine.