This would only be possible on rooted devices, and even then it could take some considerable effort depending on the manufacturer. The way call audio is routed on Android is SoC specific and often a pain to intercept or alter.
Maybe you could use something like Ubuntu Touch and utilize pipewire for the audio routing? Edit: I guess you should be able to use a script to route the call audio through to a Voip client running on Ubuntu and vice versa.