If you provisioned an inbound phone number but didn't end up using it for some reason, you can delete the number like this.
First, navigate to the Inbound Phone Numbers page in your account (this page is only accessible to account admins).
Second, find the phone number you want to delete in the table.
Third, click the action icon on the right side of that number and select "Delete Number." A confirmation dialog will appear asking you to confirm your decision to delete the number. Click "Delete" and you're finished!