This is a great project writeup, I did something _very_ similar for my son. (HT802, asterisk, twilio, calling relatives) With all of the NAT involved I could never get sjsip to work properly so I ended up having to use the old sip module, but looking at yours makes me want to revisit it.
Once you have asterisk set up and running, it becomes easy to also set up all sorts of other extensions like "check the weather" / "tell a joke" / "check the train statuses". I put up some code for it here: https://github.com/mnutt/rotary
Yes it opens a bunch of doors for the user to interact with various APIs through voice (I want to hook it up to home assistant soon too, so we can be fancy and call the "butler" and ask it to turn the lights off, etc)
Once you have asterisk set up and running, it becomes easy to also set up all sorts of other extensions like "check the weather" / "tell a joke" / "check the train statuses". I put up some code for it here: https://github.com/mnutt/rotary