You can get this directly from the Mitel System Administration & Diagnostics program, select your phone system and go to the Reporting tab, select Database Reporting, then Get new report backup, and after it completes go to Standard Reports and select Phones. If you want to get more "user friendly" report, you can select "All Extensions" then change Include to "Phones - All" and change Exclude to "All not specifically included".
If you mess around with reporting, you can get all sorts of nice reports.