Configuring Grandstream HT503 As Trunk

I've got two HT503s for which I'm trying to configure the FXO ports as trunks. I've setup two trunks in FreePBX and configured two usernames and passwords. But for one of the HT503s I see in the Asterisk CLI a repeating message:

NOTICE[1584]: chan_sip.c:28353 handle_request_register: Registration from '<sip:usernameIset@ht506hostname>' failed for '10.0.2.22:5062' - Wrong password

I do not get this from the other HT503. I have set and reset the password in both FreePBX and in the HT503.

Maybe related, the home screen on both HT503s continue to show "Not Registered."

Any Ideas?