If your concern is "muh phone number", then you can pay and not have to give the phone number to sign up.

It's already the case today (and has been for years) that you don't need to give strangers your phone number to chat on Signal. My username is soatok.45; try to get my phone number if you can.

If you want absolutely no info to be collected, ever, and there to be zero cost on the end user too, be prepared to welcome your new spam overlords. Because the people who will benefit the most from a zero cost signup that only requires a username are spammers.