As far as I know, it is not currently possible.
Personally, I’m not holding out hope for it, because it seems to me that it may be disproportionately complex to implement, compared to the benefits, and it might require a hardware configuration that the Librem 5 does not have. With that said, I am not deeply involved in the project and so I night lack insight into the feasibility of such a feature.
I started a thread about WiFi Calling some time ago.
It seems to me that it could potentially be easier to set up a workaround for the lack of WiFi calling on the carrier side than it would be to implement it in the Librem 5. For example, use a number registered with a VoIP service for all calls and SMS and then use a VoIP to GSM bridging service to facilitate “native” calls, where that might be desirable for cost or signal reliability purposes.