I built a DIY system like this using Asterisk, Raspberry Pi, and a Grandstream ATA: https://github.com/cancio/rotary
I based it on the instructions provided by this Show HN post: https://news.ycombinator.com/item?id=39801991
I based it on the instructions provided by this Show HN post: https://news.ycombinator.com/item?id=39801991