An IP telephony system allows users of the IP telephony system to register extension telephony devices with the IP telephony system. An extension telephony device is one that is provided with service by a separate telephony service provider. Once an extension telephony device is registered, a user can obtain communications services from the IP telephony system using the extension telephony device. An extension telephony device may be tied to a user's main telephony services account with the IP telephony system such that when the user obtains communications services from the IP telephony system using an extension telephony device, the user will be billed for those communications services through the user's main account.