To remove or delete a fax number from your account (we call this releasing a number) follow these steps:
Go to the numbers screen on the left and click the settings icon on the right, associated with the number you'd like to remove.
This will take you to your number settings screen. Click the red "release" button shown below
You will get a confirmation window from here, simply click "release" again to confirm. The number has been removed from the account.