I discovered this a while ago, and I have yet to be contacted by vendors about this. I am making public CVE-2022-23835: A security analysis of Visual Voicemail
Detailed analysis: https://gitlab.com/kop316/vvm-disclosure
Thank you to CERT/CC for your help in trying to contact the vendors, in CERT/CC, it is filed under VU#383864.
@zachdecook
iPhone apps, yes. But this is a system thing, and cell carriers use SMS for other network maintenance too (Cell Broadcasts are actually a special type of SMS!).