Panasonic KX-TD194 Message Waiting Unit: Lights message waiting lamps on standard single line phones Supports up to 16 single line telephones with message waiting lamps One unit may be used on the KX-TD816, Two KX-TD194 units can be installed on a single cabinet KX-TD1232 system and four can be installed on a double cabinet system for a total of 64 telephones Useful application would be on single line telephones on the system that use voice mail and require message notification.