The technology you need to comply with emerging codes. Telephone line status verification is a requirement of ASME A17.1b-2009 for new or altered elevators. (Section 188.8.131.52.6)
The PLV-20 is designed to detect faults and disruptions in telephone service and provide visual and audible local alerts when a problem is detected. Audible alerts can be silenced. Visual alert automatically resets when phone line is restored.