The RA-15 alerts the user that the telephone handset is "on hook" but the telephone has not been removed or unplugged from the telephone line/socket.
INSTALLATION: Remove the telephone top cover. Remove the protective layer from the RA-15 mounting tape. Allow the sound to escape by mounting the assembly over the largest hole available in the telephone base. Connect the two white wires to network terminals L1 and L2 or whatever terminals represent the incoming line. The RA- 15 is not polarity sensitive.