DO NOT initialize the ATA and plug it in the Ethernet as advised by Vonage at the beginning. That way the box will hit their server and download via TFTP a locked firmware (if not already installed on warehoused item).
This is the normal factory reset procedure:
From the FAQ
a) Take the phone off hook.
b) The red button on the top of the ATA-186 will illuminate.
c) Press the illuminating red button on the ATA and dial 322873738#. (The numbers spell FACTRESET# on the telephone)
d) Voice prompt will ask you to dial * to save changes you have just made.
e) Press * on your phone’s keypad.
f) Hang up the phone.
I have now heard that appearantly the most recent version of the Vonage package CANNOT be factory resetted. Hang on dslreports.com where the community discusses such issues.