Still like to know why changing sockets mad the final difference?
After eliminating the cable:
- Semi-shorted outlet
- RF Interference along the wire run leading to the bad socket
- Damaged insulation in wire run leading to the bad socket
- Bad connection to demarcation point for that socket
- Dirty outlet contacts
- Loose outlet contacts
- "Just one of those things"
It's endless. If replacing the outlet doesn't cure the problem it's likely something in the wire run. Often it's something that didn't used to be a problem until the newer generation device couldn't get its required level of line quality. There's also a good chance something just recently happened to that wire which merely coincided with your needing a new phone. Hard to positively determine a cause without some fairly expensive test equipment however. And hardly worth it if you don't need to use that length of wire.
FWIW, I had a similar experience with phones in a new client's office. Everything worked perfectly except for one outlet located about 50 feet from the phone closet. We replaced the outlet, but to no avail. A quick check on the line showed it had a marginally higher electrical resistance than it should have. But nothing that looked to me (admittedly an amateur when it comes to phones) too far out of the ordinary.
The "phone guy" we brought in however, said: AHA!
He plugged a line analyzer into the circuit and got a report back that the wire run length was about 400 plus feet. We looked up in the ceiling and found a knotted ball of phone wire about the size of a soccer ball jammed between the wall and a half dozen electrical lines powering the fluorescent ceiling lights. All we could guess was that whoever installed that line had somehow managed to knot up their roll of cable in the middle when they were pulling that run. Only afterwards did this person realize the clean lengths of wire wouldn't stretch to reach both ends of the circuit. So rather than run a whole new line, he just punched down both ends, left about 325 feet of kinked mess in the middle, tucked it out of sight, and called it done.
Once that copper "beachball" got removed, the jack now worked properly. And entire Trixbox phone system started working much better than it had previously. Apparently that mess was acting like a coil and causing all sorts of intermittent noise problems for the phone server.
Our client just assumed that their occasional phone problems were a trade-off they had to live with since they were using the "community edition" of a FOSS-based product.