Contact Billing link should open a webform to submit email to billing department
If you go to My Account >> Contact Billing, in some systems it will open an email window where you can send an email to the billing department. This only works if you are using an email client. If you are using a web based email client this is not useful.
If you had the link take you to a form where a message would be entered and then submitted to the billing department, this would be usable by all clients.