IIRC, if you've created an account from your IP address or use a VPN (which virtually guarantees that an account has been created from that VPN's exit node), they force you to go the SMS route. Apparently, there's also a requirement for Tor users.
It's unfortunate because it means there's no way that a ProtonMail account can't be tracked to some static identifier (actual IP or phone number).
That still provides a way to geolocate you. It's incredibly difficult to create a usable Email address that cannot be traced in any way (i.e. purely from a Tor connection without giving any personal information).
You can either receive an SMS to your phone number, or donate with your card.
Probably related to me trying to signup via Tor.
<quote>
Too many ProtonMail accounts have been created from your connection.
Thus, we are requesting additional verification to ensure you are human and not a spam bot.
Because Tor is frequently abused by spammers, this check may be triggered because of the Tor exit node you are using.