VoLTE does not work with Deutsche Telekom

What problem did you encounter

VoLTE does not work with my Librem 5 in the Deutsche Telekom network in Germany.

What is the actual behaviour?

VoLTE does not work. When doing or receiving a call the network drops from 4G to 2G during the call. There is no 3G network in Germany anymore...

• I'm on newest byzantinum. All patches applied.
• The problem exist since I received the Librem 5. VoLTE never worked with Deutsche Telekom (so its not a regression).
• I asked my MVNO (PennyMobil via Congstar), if I'm on a VoLTE "block list". They said no and I should check my APN (see below) settings. It doesn't make any difference using this one or internet.t-mobile...

Name: Congstar Internet
APN: internet.v6.telekom
Proxy: keine Eingabe
Port: keine Eingabe
Benutzername: telekom
Passwort: tm
MCC: 262
MNC: 01
Authentifizierungstyp: PAP
APN-typ: default,supl

• I updated the modem firmware together with Joao, it did not helped. My current firmware:

$ sudo socat - /dev/ttyUSB3,crnl
AT+BMSWVER
+BMSWVER: M100E_YCSN0_1.0.0_220926,YCSN0_M100E_1BAD_3117_V1.0.0.2_20220930,M100E_1.0.4_200715

• When using a Vodafone Deutschland SIM-card VoLTE does work out of the box with the currently installed firmware.