I’m afraid the answer above is not entirely accurate, at least when it comes to Protonmail not requiring phone verification.
While it’s true that Protonmail doesn’t require phone or SMS verification when you usually sign up for the service, it still utilizes SMS as one of the necessary verification options.
According to ProtonMail’s Knowledge Base:
“To prevent the creation of accounts by spam bots or human spammers, ProtonMail uses various human verification methods. You may be asked to verify using reCaptcha, Email, or SMS. We have an intelligent algorithm that determines the required verification method based on several factors.”
In other words, if ProtonMail’s algorithm determines that you’re a spammer, it will ask you for SMS verification. That’s supposedly done to prevent bulk signups.
However, some users have complained that the algorithm has flagged them even though they don’t have another account already.
The bigger problem happens if you are trying to use ProtonMail over a Tor network or a VPN. In that case, Proton will require SMS verification, and there doesn’t seem to be a way around that.
This is an obvious privacy flaw on ProtonMail’s side. Any data above the username and password the user needs to provide to use an email service can be misused.
For instance, a hacker can potentially spy on your text messages and calls using an SS7 attack, which exploits the vulnerability of the SS7 protocol used since the ’70s by the mobile phone industry.
I recommend using Sympa instead if you want to get an email without phone verification. Unlike ProtonMail, this email service won’t ever ask you for SMS or email verification. All you need to sign up for it are username and password.
This secure email service combats spam instead by requiring an invitation code for free accounts. You can only get one from an existing paid Sympa member or by contacting the provider. This is far better than requiring email or SMS verification and betting your data on mobile carrier security protocols, which are sometimes ancient.