The numbers are read from the SIM. If that is incorrect for your country please file an issue at https://gitlab.gnome.org/GNOME/calls/-/issues
“Custom emergency contacts” are a different matter (see the emergency info plugin in phosh for that) as they need an unlocked SIM, etc.
Also https://gitlab.gnome.org/GNOME/calls/-/issues/?label_name[]=emergency%20calling might be of interest.