“When you use Signal, your data is stored in encrypted form on your devices. The only information that is stored on the Signal servers for each account is the phone number you registered with, the date and time you joined the service, and the date you last logged on.”
You can also use their CLI app to send yourself all sorts of notifications from all your selfhosted services. The only really irritating thing is that they won’t provide an arm binary, and compiling the app on arm is… decidedly not easy.