That command will take you to enable mode, it might ask you for password, if so then the default is password
If that doesn’t work and you can’t guess it you will need to default the unit from bootstrap, direction in link:
Once you get to bootstrap, then you can do the commands below:
If it doesn’t ask you for password it will take you to the TA908# prompt, then you can enter this:
Erase startup-configuration? y
Startup configuration erased.
Save System Configuration?[y/n]n
You are about to reboot the system. Continue?[y/n]y
Let me know if you have any further questions.
Note that Mark's answer will erase the configuration but not technically "default" it. Units from the factory have a default configuration with certain parameters turned on such as a login banner, default subnet and DHCP server, web server enabled, default user/password, etc.
In our shop we do exactly what he recommends above on brand new gear and anything to be redeployed in order to ensure starting from a clean slate. If you're comfortable starting from scratch on the command line, go for it, in my opinion this is best practice to remove any cruft. If you expect to be able to log in with adtran/password, get a DHCP address and have the web GUI enabled you'll need to add the default configuration back in.
For Gen 1/2 go here: https://supportforums.adtran.com/docs/DOC-5917
For Gen. 3, here: https://supportforums.adtran.com/docs/DOC-7207