I do not know if the A400 0r A800 is compatible with the ATA.
I tried several cheap voip devices with my tenor ,and only few worked.
At the same time, when I tried those phones with gnugk, that it is free, it worked like a charm.
Be sure that you do not have more than four registrations already on the gatekeeper of the A400.
That is as much as it takes when you buy it without upgrade.
To verify the registration go to the Tenor and type GK EP. That will give you a list of registered endpoints.
If you have more than four, you could still try a static route to the Quintum. They do not count as registrations.