[22c]

I wouldn't say it's an impossible problem. It's fairly simple, in my mind.

If someone tries to send me a message on Signal it should go into purgatory. On my end, I should be able to see who is trying to send me the message (yes, including their phone number, given that is how Signal has decided to uniquely identify users) and I should be able to see what their public key is. Then I should be able to either accept that message, which would essentially make my presence on Signal known to the other party, or choose to first verify that the public key matches that of the other party via the existing "in-person" verification method.

Alternatively, I can leave the message in purgatory where a message from someone I don't trust belongs and eventually times out. Not only do I never see the contents of the message, but the sender of the message will also never know if I am on Signal.