One way / garbled audio

Hello,

I am new on this forum and also quite new to FreePBX, so please bear with me if I am asking a FAQ.

I have a FreePBX box running with a PJSIP trunk to may SIP provider 1und1 (Germany).
I also created two PJSIP extensions, 100 and 200, which i can access with my two phones running Acrobits Groundwire. Woks fine so far, extensions can communicate with each other and can also be called from outside via Phone1 -> Sipgate -> 1und1 ->- FreePBX -> Phone2
Problems arise when I call from outside via Mobile Network. Phone1 uses German Telekom, Phone2 uses 1und1, which in turn uses Vodaphone (I believe).
In order to test things, I setup the inbound route to Voicemail / Ext. 100 with messages emailed to me. Phone2 works fine, I can hear recorded message from my mail program.
When dialing in with phone1, I do net hear anything, no busy message, no recording prompt. If I blindly speak a message anyhow, it is recorded and emailed to be, but it is totally garbled.
Is this a codec problem?

My codecs list: ulaw, alaw, gsm, g726, g722
Asterisk Version: 16.13.0
You can access the garbled message here:

[Garbled message]
(https://www.rebehn.net/FreePBX/msg0002.wav)

Thanks for any help,

Heinrich