Router PSTN connectivity is generically referred to as voice gateway functionality, offering a gateway for voice over IP (VoIP) calls to, and from, traditional analog or digital PSTN or private branch exchange (PBX) calls. You can use a router voice gateway to connect to PSTN central office (CO) switches, private branch exchanges (PBXs), Key Systems, time-division multiplexing (TDM)-based interactive voice response (IVR) systems, traditional TDM-based voice mail systems, and any other legacy (non-IP) voice processing or telephone equipment.