In theory this could happen.
Voive-over-WiFi (VoWiFi) aka WiFi-Calling is an open standard and can be implemented as free software.
However, it is also a very complex standard that is very difficult to implement (as is VoLTE which is closely related and based on the same software stack).
I don’t see a free IMS/VoLTE/VoWiFi-stack for phones coming any time soon; if the Librem gets VoLTE, it will be a proprietary implementation in the blackbox modem, which right now is the only reasonable way to do it - since the WiFi-module is separated, it will not be possible to include this.
Additionally there is the issue of carrier certification: mobile network carriers have stringent quality requirements, and they tend to whitelist only certified implementations (which doesn’t necessarily exclude free software implementations), because they get blamed for poor voice performance (drops, noise, other issues).
This hurdle would have to be passed as well.
Again, I can see this happening, but not in the near future.
As it stands, there are no open implementations even for android which relies solely on proprietary baseband-firmware in combination with proprietary userspace support.