fizzo, I was able to make Visual Voicemail work on my #LinuxMobile phone (OnePlus 6 running #postmarketOS ) with Iliad Italian operator.
I had to guess the config phone number, I used one that was sending me an SMS on new voicemail and it worked! (VVM was previously enabled on an iPhone)
@kop316 did a wonderful job with vvmplayer! it's well documented and was relatively easy to gather the missing pieces.
Bonus: After this MR gets merged, VVM will work out of the box on Iliad https://gitlab.gnome.org/GNOME/mobile-broadband-provider-info/-/merge_requests/112