The phosh-mobile-settings app now allows customizing the phone call ringtone, SMS, and instant messages alerts as of 0.27.0. It can be found under the Feedback tab. Only OGG files are supported right now.
Changelog: https://gitlab.gnome.org/guidog/phosh-mobile-settings/-/blob/main/NEWS
Supported formats: https://gitlab.gnome.org/guidog/phosh-mobile-settings/-/merge_requests/50/diffs