GSM & PSTN Double Network Alarm System
this is a good product for alarm use

GSM & PSTN Double Network Alarm System

when PSTN line is lost ,alarm out though GSM network;when GSM network is interrupt,alarm out though PSTN network. normally, alarm out will secter PSTN line for lower cost,and SMS will sent though GSM network if needs.
this is a good product for alarm use.
Thanks for you care for.