Missing dial tone when "use voLTE" is activated

/e/OS Devices Fairphone
1

When the option to use VoLTE is activated (for improvement of sound) there is no dialing tone on outgoing calls; there is silence instead until th called person accepted the call.
When the option is disabled (using the 5G connection), the dialing tone is hearable.
I had a conversion with the T-Moble service about this issue, and they came to the conclusion it would be a matter of the custom ROM ( /e/OS). The guy from T.Moblie mentioned, the dialing tone is generated by the device itself − it sounds strange to me, but I’m not a not a smartphone technician and thus, cannot judge that statement.
However, I would like to use the VoLTE but it is irritating missing the dialing tone.
So I’d like to ask the /e/OS team th check how this can be solved.

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2

Not sure it’s an “issue” in fact. I notice that I don’t have dial tone neither when I’m connected to the WiFi (while WiFi calling is activated). I’m on Fairphone 4. Idem for my spouse.
My step mother who uses LineageOS 18.1 on Redmi Note 7 has exactly the same behaviour.
Note that we are all on “Salt.” carrier in Switzerland. This can also be related to the carrier MSC profile configuration (MSC = Mobile Switching Center)
So I guess it’s rather a LineageOS thing (on which /e/ is built) or a provider configuration. But I might be wrong…