The distinction of what belongs to telecoms and what belongs to the Internet is only due to how the technologies have evolved. It's a legacy quirk of the "it's always been like that" type. I'm all for reducing telecoms entirely to "dumb pipe" ISPs and moving
everything to IP, calls too. Mandated inter-connectivity only exists to fix a problem that telecoms have created by having access to unencrypted insecure special-purpose comms traffic and being able to abuse this privilege. This should have been encrypted IP data they can't touch or discriminate.
Phone numbers are an outdated idea, and shouldn't exist any more (and Signal is terrible for using them). You should have more privacy and have more control over your identity than what the legacy telecom setup allows. For example in most countries in Europe you can't get a phone number without a government ID tying it to your legal identity, and telecoms may be obliged to log your call metadata. Would you prefer e-mail as an open IP protocol, or a setup of traditional licensed postal operators that require government's permission to make an e-mail account for you?