For removing the old number from Amazon you can simply follow these;
Click on the “Accounts & Lists” button in the top right corner of the page, then select “Your Account” from the dropdown menu, then go to the “Login & security” section, and click on the “Edit” button next to “Phone number.” click on “Delete” next to the old phone number you want to remove.
For seller central you can follow these steps;
Click on “Settings” in the top right corner of the page, select “Account Info” from the dropdown menu, click on “Business Information” in the left-hand menu, and click on “Edit” next to the “Contact Information” section, locate the old phone number you want to remove and click on “Delete.”