[Biggles]

If it is a feature phone, it may have four wires. Typically on our mini-switches such as Panansonics the feature phone has extra functionality and works on four wires. The rest of the extensions are commonly two wire working but I believe they can be configured to either two or four wire working in the switch. The majority of our networks are plain old two wire working, and I personally don't have a lot of experience of four wire feature phones except I believe there may be one data pair and power on the other?
Alan.