I have a FritzBox before and a Snom IP dect station behind my IPFire. The FritzBox is the SIP server for the dect station. After update to core update 156 no telephony is possible. An outside caller hears the ring tone, but it doesn’t ring. An inside caller hears nothing.
You have to forward the SIP and RTP ports of the snom. (Before the update this was done by a sip application layer gateway, but we had to remove this because it breaks security.)