Is it supported to have remote offices connected via VPN Gateways? Has anyone gotten it working?
Yes, you can connect phones at a remote location to your SPA9000 though a VPN connection.
If it's connected via VPN then you must treat the "remote phone" locally registering with the SPA9000. Connect the WAN port of the SPA9000 at the back of the local router, the WAN should be getting a local IP automatically from the router's DHCP which is 192.168.0.xxx, you may want to set the Internet Connection type of the SPA9000 to static and just assign a valid IP address on it.
On the SPA9000, under WAN , set Proxy Network Interface to WAN. This should let the "remote phone" automatically register with the IP PBX.