Have looked all over net for solution to this common problem . Tried various things to no avail.
NAT enable and keep alive enabled. Reg changed to 180
Calls show up in voicemail and missed calls list.
This is working now. The problem was that the Forward to Chat was not checked. Checked this box and all works fine.!!
You may also have to do port forwarding on your firewall in your router/modem, etc.