You can actually do voicemail within Twilio for free using their TwiML Bins and Functions, you don’t need any extra hardware to maintain.

Just did this for a couple of numbers I set up with SIP. The docs were... confusing and inaccurate but I finally got it to work.

No write up unfortunately, I actually made it a few years ago as I was getting a lot of recruiter spam calls, but ever since the GDPR came in that has stopped. I don't receive that many calls these days.

I have my (cell) phones voicemail set to my Twilio number. When a voicemail request hits Twilio, it calls an endpoint hosted on my Pi, which then just calls the Twilio APIs to

1. tell the caller to leave a message after the beep

2. record the message

3. use the speech-to-text API to transcribe the message

4. send me an SMS message with the transcription

It's really just an ersatz visual/text voicemail service that I think iPhone users get. Also the Twilio speech-to-text transcriptions are hilariously bad, I don't think it copes well with UK accents :)