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?