I just did. I don't know exactly which one of this steps made it, though.

First, I added the name of the SIP user (usually, the number of the line) as the "Owner User ID" in the device configuration and checked the Media Termination Point Required box.

Then in Line Configuration I added the same user to "Users Associated with Line". And finally, I went to the user and added it to the Standard CCM End Users and Standard CTI groups.

I was just toying around with settings because I had the same problems. IMO, since the N800 disconnected receiving the call, it could have been a signaling problem, so the Media Termination Point is what I bet the problem.