Fritz!Fon App over Wireguard: no voice/sound when I'm in school, even though wireguard itself works

Hello! I'm not sure if it is a right place to ask the following question, I also posted the same question to Unifi Forum, but since it involves Wireguard on OpenWRT, I'm going to post it here, in case someone might know how to solve my problem.

I have Telekom VDSL, Fritzbox 7530 AX, and EdgeRouterX with OpenWRT 22.03.3 behind it. I installed Wireguard and kmod-nf-nathelper-extra on ERX. My Fritz!Fon App on Samsung A3 (2017) works fine over wireguard, when I'm on a "usual" public wifi. It seemed like, whenever wireguard generally works, Fritz!Fon also works over wireguard.

However, when I'm at the elementary school where I have something to do, wireguard itself does work in general: I can browse Luci, Unifi Controller, etc. However, Fritz!fon App doen't bring any voice or sound. When I call someone, it does ring their phone, but they don't hear my voice. On My Fritz!Fon, I don't even hear a calling tone, it just shows on the display that it's connected. If I call my Fritz!Fon number from another phone, it won't ring: on the fritzbox it shows as a missed call.

At this school, they have Telekom Fiber Optic, Digitalisierungbox, and Unifi Security Gateway, and a bunch of Unifi APs. So it's double NAT.

I'm wondering if this problem has something to do with their double NAT, just because I also have double NAT, and without kmod-nf-nathelper-extra, the voice goes only in one direction. But since I don't really understand how wireguard really works, I am not sure if it really matters. Another possibility is that their firewall blocks voice traffic, but I don't really see how that's possible, or meaningful.

I would appreciate your input!

It turned out to be a bug of Fritz!Fon App.,127714.html
It was fixed already a year ago. I bought my smartphone after they fixed the bug, so I don't know why I ended up with the version with the bug, but as I updated to the current one, I was able to use it over wireguard also at the school.
I thought about all kind of reasons why it didn't work, but I didn't think of this one....

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