Earlier quoted context omitted.
Pay $5 via crypto, venmo, whatever to add a similar cost barrier. Make it separate from nitro.
The difference though is that verification through phone numbers relies on money that you've already spent, which is a lot more reasonable for the majority of users. People would still be unhappy if they had to pay 5 bucks if they didn't own a phone.
Also there's precedent of sites doing this. MetaFilter charges $5 to create an account. The bitcoin wiki used to require a fee (in bitcoin)[1]. Something Awful forums have a fee.