These are some common tools used to work on this device. You might not need every tool for every procedure.
Identify what needs to be done to fix your device with the troubleshooting page.
The Inter-tel 8660 can be identified very easily. There is an Inter-tel label on the front of the phone and a label on the bottom of the device that clearly states the Model Name: 8660. The 8560 model is very similar to the 8660, but the 8660 can be differentiated by a few simple features. The 8660 has additional ports on the back that are used for Internet and power connections, does not support the Off-Hook Voice Announce feature, and does not support the Enhanced Speakerphone Mode feature.
This device was created as an upgrade from previous phones such as the Axxess Executive Display and the IP PhonePlus. It has to ability to place intercom calls, place outside calls, place conference calls, forward calls, and page. There are also many other features that come into play especially when the device is hooked up to a network in an office.
The Inter-tel IP 8660 user guide.
Technical support from Inter-tel (now known as Mitel).
If you can't fix your device on your own, there are also repair services offered by Mitel.