The phone number of Proton Mail Customer Service is unavailable. However, a support form is within easy reach to submit your questions to the Proton Mail customer service. Send in the issue that you are facing with your Proton Mail along with your proton username and the computer operating system on which you are facing the issue with. You can also send a direct email to the support team at firstname.lastname@example.org. For other critical issues like your email account has been hacked, a different user has tried to gain access to your email, etc, contact the Proton Mail customer service at email@example.com.