Any SIP compatable device is capable of being used on the system, but there is a level of fully supported and integrated versus being able to be used with the system.
Fully supported devices include the following:
Linksys SPA series Phones and ATA devices
Cisco SPA series Phones
Other Devices that support SIP protocol can be used, but they will not be able to be automatically configured (provisioned). These will require you to have access to all the settings, and make some entries to configure the device manually