T22P call quality issues with long-distance conference calls

[JoeM]

We recently installed ~50 T22P phones for a company using 3CX, and have received numerous complaints about call quality, specifically with long-distance conference calls. I have been told, and have reproduced, good call quality to/from extensions. When calling some numbers (roughly 1/3) outside of our network, I can only hear bits and pieces of their side of the call, and it is the same for them. All long distance conference calls are like this. I've been playing with settings for two weeks without success. The firmware is up to date (7.73.0.40) Any ideas?