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The TA600r is not an AOS product, so the "bypass password" procedure does not work on it. To get into bootstrap, you can hold down the "b" key while powering it up. This will allow you to upgrade the FW but little else. If you want to factory default the unit, you can hold down the "f" key on boot-up.
At the LOGIN prompt, have you tried just hitting <ENTER>, or using "password"?
If you do not want to factory default the unit, there is a challenge/response procedure you can go through to restore the default password (which is "password"). You will have to call ADTRAN Support (888-423-8726) to go through the challenge/response procedure.