The plug-in PSTN module enables the panel to communicate with a central station (ARC) using common format protocols (SIA, Contact ID). The PSTN module can be used as a primary communication channel or as a back up to the Cloud connection. in case of communication failure. The module should be plugged into the GSM/PSTN module slot on the LightSYS+ panel. In installations that require both PSTN and GSM modules, the GSM module should be installed on the COB (Cellular on Bus).