Fritzbox only knows the IP networks 178 and 188.
This makes network segmentation almost impossible.

Setting up as an Internet router is relatively easy on the Fritzbox.

First, the device should be reset.

Select "Via an external modem" on the first run wizard.

The Fritzbox automatically fetches the address range 192.168.188.0/24

On the OPNsense this address range must now be set up as an interface.

Should the Internet work via "WAN", telephony can be set up.

Now comes the trap: Although everything is displayed correctly, and outgoing and incoming dial tone and ringing, you hear nothing.

The reason for this is relatively logical: The NAT reflection does not work. The phones behind the Fritzbox are completely detached from the OPNsense network and cannot be addressed.

The reason for this is relatively logical: The NAT reflection does not work. The phones behind the Fritzbox are completely detached from the OPNsense network and cannot be addressed.

So how do you solve this?

The Fritzbox needs a manual outbound NAT rule.