I’ve done it recently but don’t remember all the steps… I used a proton mail account. If I recall the account will get started and then shortly thereafter they will ask you to verify with a phone number. If you don’t your account gets suspended and every time you try to login they continue to ask. Just refuse but contact customer service and tell them you don’t want to verify via phone number. After a couple of weeks they will unlock your account.
Not sure if I have the order but the gist is to register, refuse, and contact customer service. Then wait.