Yeah, the dialer is not in a good state right now. There are several issues open in the developers git repo.
I did however just find out, that if you switch to pipewire with:
sudo pacman -S pipewire-pulse pipewire-alsa
(say Y to replace) and reboot, you get “working calls”.
The lockscreen thing still doesn’t work and audio is hit’n’miss, but at least I can hear stuff on both ends.