There isn't a built in backdoor password for the 2k.
Someone may have set one up but it doesn't come from the factory with one.
Mitel can reset a password but the password is based on a calculated algorithm that changes each day.
(Ok, so maybe that is a backdoor password) Mitel isn't sharing what that password is based on so for us mere humans we have no way to get in.
You can try the default users:
system
installer
maint1
maint2
Ralph