I’m not familiar at all with the internals of Skype. It may not even be VoIP, as VoIP is understood in the open world.
You should hunt around on the internet to see whether anyone is able to use a generic VoIP client with Skype.
My suspicion is “no”. I think Microsoft changed something a few years ago so that even if it used to work, it will no longer work. But don’t let me dissuade you from investigating further.
I have my doubts that Skype is available as a native Linux client for ARM i.e. that would run on the Librem 5. So your only option, if you absolutely insist on using Skype, might be to get Android emulation going on the Librem 5 and then use the Android Skype client.
Needless to say that running blackbox Microsoft code on your Librem 5 requires a certain amount of doublethink.