If there's interest in this kind of stuff, I'm working on open-sourcing the core of a SaaS side project[0] I have. It's visual voicemail on steroids (cliché, sorry) and, like the linked article describes, accepts your forwarded call and handles the voicemail. Afterwards (and this was my original use-case), it can trigger a bunch of hooks to your favorite things — Dropbox, email, Slack, SMS, Zapier, what have you (and all right out of the box).
I built it because I was on Android and wanted to keep my moms voicemails. :)
This is interesting. Let me give you some potentially helpful context:
Some services require companies to have a public phone number. This doesn't need to be a real support phone number, but it shouldn't be completely unattended.
Your service would fit right into that, and if you (optionally) handled the purchase of the number itself (Twilio will do that), it would make it quite literally a 5 minute setup.
The only other thing I haven't found is a forwarding service, that would allow such companies to purchase a number in a particular country, to forward calls/sms to another arbitrary phone number (or an email/voicemail box). I'd certainly use that for myself as well.
I would be interested in seeing it if open sourced. I used YouMail in the past and their UI really bugged me, so I was playing around with the idea of building something more modern and self hosted.
I built it because I was on Android and wanted to keep my moms voicemails. :)
[0] https://tinyvoicemail.com