T46G different volumes

[Headon_Support]

Hello,
we have here some troubles with some T46G devices:

We have a scenario at a customers place,
At the Moment he has running 2 PBX's in the same Network,
the routed toghether with BeronetBoxes.

In some cases, we have the Problem, that the audio is very quiet.
I have here some cases:

TO QUIET [412] -> T46G -> 3cx -> Berofix -> Octopus -> [33]
TO QUIET [412] -> T46G -> 3cx -> Berofix -> Octopus -> [507]
OK [423] -> Softphone -> 3cx -> Berofix -> Octopus -> [507]
OK [446] -> Snom 870 -> 3cx -> Berofix -> Octopus -> [507]
QUIET Handy -> Berofix -> 3cx -> T46G -> [412]
OK [423] -> Softphone -> 3cx -> T46G -> [412]

So you can see, with the snom and the 3CX Phone everything works fine,
we only have troubles with the T46G.

We've tried the following:

Set the codec in the extensions to PCMA
Set the codec in the Beronet to PCMA
We changed the following line voice.handset_send = 53

To manually adjust the volume on each call is not an option, because then you have to adjust the volume (in the worst case) after each call.

Are there any ideas, what went wrong with this phones or what we forget to configure?

Best Regards and Thank you very much
Daniel