I don't have any experience with the N810 (or any Nokia phone).

If you have a place for specifying the registration timeout, it is set
at 1200 seconds max on the gateway. If your registration times out,
the gateway won't be able to send calls to you. Most devices/user
agents won't have to set this. It is usually automatic (determined
from the server during the registration process).

I tested my own number on the gateway and received the call
successfully several times in a row just now.

Also verify that in Google Voice you only have the Google Chat option
selected for receiving calls. If you have more than one option
selected (e.g. Google Chat and your cell phone) it will not connect
reliably.

Good luck,
Bill Simon