The secret code for Nokia does nothing. Maybe it needs some unlocking first?
But I checked with Android and the Network app you suggested from F-Driod. All is functional there and if “LTE only” is selected calls go through normally. Moreover the Android top status bar says “VoLTE2”. I do not know if this “2” is what makes L5 not to work with VoLTE here…
Question here is: Does ‘LTE only’ means you are using VoLTE?
What else? In the menu of the Network app from f-droid this is the only relevant choice.
This tells me that you are using 4G for data and 2G for voice.
Thus you are not using VoLTE.
Also, despite your Nokia supports VoLTE, it does not say that it is using it.
So far, it is unclear what technique is used while you are making phone calls.
Hence my question: Are you using VoLTE or not.
Let me try to organize:
On Android when “LTE only” is selected everything works and the phone says it is connected with VoLTE2 in the status bar. This tells me that the carrier provides VoLTE. So this issue has nothing to do with the carrier.
On Nokia with KaiOs, I can not choose LTE only (it does not have this option).
On Librem5, VoLTE is enabled in the BM tool. Other than that I have no indication. If you know how to check please explain to me. How do I check?
The 2 after the VoLTE icon appears to be an indication for second SIM card.
At least that is what I can find about it on the internet.
Also there seems to be a bug for the VoLTE number indication see Issue.
Seeing you have issue’s with the 2 SIM card’s, it looks like you need to dive into the mentioned issue
(speculation).
Another thing I found about that indication: It shows that your phone is capable of using VoLTE, not that it actually is using VoLTE. See here
If a phone is set to LTE Only (or 4G Only, etc.) and the phone successfully makes calls, then that means the calls are happening over LTE, i.e. Voice over LTE.
An indicator that VoLTE is not happening is that no call can go through while LTE Only is set.
But it should change to a lower generation indicator during a call, if the call is not using VoLTE.
Also, on a dual-SIM/dual-Standby (DSDS) phone (or at least on Sony DSDS Androids) there are usually two network indicators present, side-by-side, one for each SIM. [EDIT: I’m not sure how the “twin SIM” that @antonis is using would handle the indication, though.]
Agreed
That’s something I would expect to happen too.
Reading back, it’s unclear if this is the case on the Android device and also how Android displays this when VoLTE actually is being used.
The display doesn’t change during the call if VoLTE is enabled and VoLTE is being used. Note that different OEMs, custom ROM builds, and/or network carriers may use different labels in the notifications area, e.g. “LTE,” “VoLTE,” “4G,” “4G+,” etc.
OK, since I don’t own an Android device I didn’t know.
Thanks for filling that gap of knowledge. 
Remains the question if the OP notices a changed label (if dropping to a lower generation) or not on the Android while making a call.
Indeed, but aren’t we talking about the Android with the first SIM inside as well?
It is my understanding that Antonis has two phones with two Sims that use the same phone number.
He has problems with the SIM inside the L5 (wich seems to stop functioning once in a while).
I was merely trying to understand if he is sure about using VoLTE on both phones.
Hence my questions about the Android phone.
After the sim swap so far so good. I will do the ultimate test next week, which is to use only Nokia for 2-3 days and then return to L5.
It seems that this issue gets solved if the SIMs are swapped. It is about 10 days now with no problems. So I marked the SIM swapping as a solution. Thanks.