It's not something you can avoid by simply not using a credit card. Proton can, at their discretion (such as if you use VPNs outside the imperial core, in which case it happens reliably and instantly) lock you out of your entire account to demand a phone number. Vanishingly small % of people will not have their personal identity exposed by cell data & be ready to deal with losing their account on short notice. It's the same shady tactic that Discord uses to get phone numbers from reluctant people and it should not be tolerated.