Self contained Intercom unit typically used to monitor an entrance door, the door box then sends chime tones to all extensions programmed to receive chimes, weather tight unit can be mounted outside, maximum number of DSX analog door boxes installed is determined by the number of 2PGDAD models which in turn is limited only by the availability of digital station ports, the DSX-40 has a built-in door box port, SKU: CPTLSL1353