The Distance Plus Family (DPX) is made to extend voice lines to longer distances. If your present installation length is longer than standard voice (POTS or ISDN) can spread, or your new VoIP-equipment offers less distance than your ancient equipment, the DPX does solve the problem.
The distance can be up to 6km on a TP installation, e.g. along railroads. The supervision of security infrastructure and maintenance based on voice transmission technique can be easily improved for hard to reach installations, e.g. remote repositories, solar & wind energy plants etc.