With projects like these, I’m always torn between thinking that it’s cool it’s possible, and horror that someone somewhere will try to use this in production code.

Depending on the complexity, there’s also abbrev-mode: https://www.gnu.org/software/emacs/manual/html_node/emacs/Abbrevs.html

Thanks! I found the instructions here, which don’t seem too hard: https://blog.voip.ms/voip-on-cell-phones-ios-and-android/

But otherwise, VoIP.ms looks pretty much like what I’m looking for. It also seems super super cheap for my needs (keep a phone number alive, make a few calls or SMSes per year).