Can you post a short version of what you sent them for inspiration?

A lot, but farming phone numbers from poor countries is also cheap and Signal sends them insanely expensive SMS. There is no perfect solution, spammers aren’t stupid. Since Signal is centralised they can enforce PoW incrementally if they get reports for spam, I still think it is way better than hCaptcha which is garbage.

The challenge of having your device solve a nasty PoW that takes minutes would not deter most people: a timer once is better than evil captchas, phone numbers, etc. I don’t understand why they use hCaptcha and not that.

Use BitTorrent with DHT, no problem if site goes down.

Why haven’t people learned?! It’s been decades now!