So I have played around with this a bit the past few days and was not able to get it working. Using the recommended configuration, everything appears to work, but there is no audio in the calls, and external to internal calls drop after 15 seconds.
As I stated in my previous post, I can get everything except status by manually changing the settings.
Today, I decided it wasn' t worth any more effort, and I set up a site to site VPN tunnel between the two locations. (There is a FortiGate protecting both sites.) I rebooted the phone as an internal extension, and everything worked, except the line status! I looked over the phone information, and it showed internal addresses, so it appears the traffic is going over the VPN, but still no status. Now I am completely perplexed.
Just thought I would share. I have to believe the problem actually lies somewhere in the FortiGate configuration, but I have no idea where. I think I will just change the button with line status to another speed dial.