I saw it can give you notifications over Signal. Does anyone have any info about how it sends messages without a phone number? I thought you need a phonenumber

I saw it can give you notifications over Signal. Does anyone have any info about how it sends messages without a phone number? I thought you need a phonenumber to have an account. If I can send messages without a number then I'd like to try to add some feature to my VPS so that it sends me status updates :) Can you imagine getting a notifications when a build was done or sometching like that? It'd be really handy