In order to verify a new phone number, your old number must be removed. If you need a phone number to be removed, please see the instructions on how to request it, here.
To verify a new phone number, follow the steps below:
1. Go to your "Settings" page on the left menu and go to "Verification".
2. Type your phone number and click on "Send Verification Code".
3. The code will be sent to your mobile. Enter the code in the "Enter The Verification Code" field and click "Verify". Note that the code expires in 180 seconds. If the code has expired, you can request a new one.
4. All done. Your new phone number is now verified.