SIP telephony behind a pfSense firewall

When we got the fibre connection, I decided to use Bahnhof as our service provider. They enable a SIP phone connection at no extra cost, but they don’t support using third-party SIP boxes; you have to use their combined router/wifi AP/SIP converter (a box by Tilgin), which they manage for you.
Naturally, since I’m tinkering a bit, using a third-party router I can’t manage in front of my network would be unacceptable. The next best thing, then, is to put the Tilgin router behind the pfSense box and use it only for SIP.

Bahnhof demands opening the following ports for SIP telephony to work:
69 – UDP
5060 – 5080 TCP + UDP
9000 – 14000 UDP
50000 – 60000 UDP

I set up a DHCP reservation for the Tilgin box, gave it an alias in pfSense, and NATed the ports specified above to it.
Second, I connected the WAN port of the Tilgin box to my network, and saw that it started up fine, and I could both call out and receive calls using a phone connected to the router. All fine, right?
Not quite. After a few hours, incoming calls stopped working. A couple of minutes with my search engine provided the following page:
The required fix was the first one suggested; to enable hybrid outbound NAT and static ports for UDP traffic from the Tilgin box.