Fritz!Box does not capture (RTP) traffic properly

From time to time I have to capture telephony traffic on a Fritz!Box Fon WLAN 7390.
Note: This is for research reasons only. Please do not capture calls without permission by the calling parties.

A very useful tool for capturing traffic is the built-in capture tool Fritz!Box Paketmitschnitt. You can access this via web browser: http://<your-fritzbox-IP>/html/capture.html

Unfortunately, the box happens to route some traffic past the capture. I faced this occasionally with RTP packets which definitely arrived in my local network but were never visible in my capture (except for one or two packets compared to ~1000 actually being transferred).

This gave me headache, confusion and desperation.
The solution is simple: it's called Packet Accelerator and can easily be disabled.

Log in to your Fritz!Box via telnet and do the following:
echo disable > /proc/net/avm_pa/control
This disables the PA and makes your packets appear in your capture. However, could make routing slower.
Buchcover