From Linksys manual:
To reset the ATA device to the factory defaults, perform the following steps:
1. Connect an analog phone to the ATA device and access the IVR by pressing ****.
2. Press the appropriate code to reset the unit:
• Press 877778# to reset the unit to the defaults as it shipped from the ITSP. This will reset the User account password to the default of blank.
• Press 73738# to perform a full reset of unit to the defaults as it shipped from Linksys.
This will reset the Admin account password to the default of blank.
3. Press 1 to confirm the operation.
Press * to cancel the operation.
4. Login to the unit using the User or Admin account without a password and reconfigure the unit as necessary.
