> There's no technical reason for modern instant messaging to still rely on any part of the phone network, except as a dumb pipe to transmit data over.

I understand that the reason Signal, for example, does this is to cut down on spam on the network. You can't use traditional spam filters, since that would negate the privacy aspects of Signal. So instead you have to increase the cost to spam (i.e. by requiring a valid phone number, which can also be banned from the network.)

That's not a technical reason. They could just as easily ask for cryptocurrency payment of $1 (which is more than a phone number would cost), or even proof of work. The fact that they require an identifier that is typically tied to one's real identity is suspicious on its own, IMHO.

They certainly could but that would raise the barrier to entry quite a bit. My parents wouldn't be able to do that.

They could just support both!

Non-tech-savvy users or those that want to use their phone number as their primary identifier anyway could still "pay" with SMS verification; others could pay with money and pick their own identifier.

Okay. So now the question is, who maintains this database of people who can be contacted using a phone number-less system? And what do you use as the unique identifier?

Email addresses have worked pretty well, and have the big advantage of allowing "self-custody" via using your own TLD, but don't require it.

I don't have any issues with also allowing phone numbers as identifiers and most people preferring these, but I don't want to be forced to do so myself.