We have been dealing with this issue on POTS lines and digital phones for awhile. Eventually, we got to Tier 3 support with Mitel and they acknowledge this has a known issue with digital phones and POTS lines. They informed us that they were working on a fix. Finally, they got back with us and told us to go to the FTP site and pull 6.2.5.90 from there to fix the problem. The digital phones have been performing a firmware upgrade with this version. Mitel says now that if we experience echo with the 6.2.5.90 to call technical services and they have lots of things they can do to fix this. I did have a new install complain about echo on 6.2.5.90, but I ran a hybrid loop start test and have not had complaints since.
