SNOM PA1+ DTMF not working

Hello,

I want to use an SNOM PA1+ as door opener. So far most call-functions are working fine in combination with a Fritz!Box 7490 as telephone system.

Using firmware 10.1.169.15 it is not possible to control the 3,3V Pin Output (PIN 3) via DTMF.

I can successfully control the PIN 3 via webUI and http action, but not via DTMF ("3#" = on | "3*" = off)

In the Snom SIP-Log (see below) the recieving of the DTMF is visible, but no action is performed. 

Where else should I perform settings or where can I investigate further to get it working?
Is there maybe a bug in the firmware?

Received from Udp:192.168.X.X:XXXX on Udp:192.168.X.X:XXXX at Jun 8 20:42:10.723 (586 bytes):
[...]
User-Agent: AVM FRITZ!Box 7490 113.07.57 (Sep 1 2023)
Supported: 100rel,replaces,timer
Allow-Events: telephone-event,refer
Content-Type: application/dtmf-relay
Content-Length: 24

Signal=3
Duration=100

Sent to Udp:192.168.X.X:XXXX from Udp:192.168.X.X:XXXX at Jun 8 20:42:10.735 (372 bytes):

SIP/2.0 200 Ok
[...]

Received from Udp:192.168.X.X:XXXX on Udp:192.168.X.X:XXXX at Jun 8 20:42:10.904 (586 bytes):

[...]
User-Agent: AVM FRITZ!Box 7490 113.07.57 (Sep 1 2023)
Supported: 100rel,replaces,timer
Allow-Events: telephone-event,refer
Content-Type: application/dtmf-relay
Content-Length: 24

Signal=#
Duration=100