I now have internet connection w/my Fritz!box. I also have the Fritz!Box set to a Static IP in the OPNSense firewall DHCP. I've setup the port forwarding exactly as you described for only the Fritz!box entries (Total 4 entries under Firewall:Port Forward). My Fritz!fon now only has a busy signal when I attempt a call. Is there something I'm missing for the Fritz!Box to work as only a VOIP engine? Here is a screenshot of the configuration on OPNSense.
Also, I read elsewhere in the pfSense forum that another person had to setup two firewalls in order to fake the Fritz!box into having it's Public IP rather than a private one when used under OPNSense. Do you have a similar setup?