The number in the “My Account” page in the developer portal is used for a number of account verification purposes. This number is associated with the current user’s Amazon customer account and is user-specific. Because this number is verified through SMS text, the added number must be a mobile phone number.
Keep in mind, adding a number to "My Account" is not necessary if your account is not monetized and you don’t plan to monetize in the future.
Each mobile number can only be associated with one Amazon account. Currently, there is no way to add the same mobile number to different accounts.
To remove a mobile number from an Amazon.com account, sign in and hover over the “Account & Lists” section. Choose “Your Account” then the “Login & security” option. From here, you can see and edit your account information. To remove the mobile number, click “Edit” next to the number and choose the “Delete” option.
If you are experiencing issues making updates, please contact Customer Service at: https://www.amazon.com/gp/help/contact-us/general-questions.html