KX-TD170 adds 8 multi-line digital ports ("extensions") PLUS 8 single-line analog ports. The KX-TD1232 can use two; the KX-TD816; just one. It is a large "pod" that attaches to the front door; with a flat ribbon cable that goes through the door and plugs into a connector on the front panel of the control unit. Phone connections are on a 25-pair female Amphenol connector.