Cisco medium-density Analog Voice Gateway provides enterprises, managed services providers, and service providers the ability to directly connect public-switched telephone networks (PSTNs) and existing telephony equipment to Cisco 4000 Series Integrated Services Routers. These FXS and FXO ports provide Dual-Tone Multifrequency (DTMF) detection, voice compression and decompression, call progress tone generation, Voice Activity Detection (VAD), echo cancellation, and adaptive jitter buffering. Cisco VG400 Voice Gateway is a medium-density analog voice gateway. It is an intermediate path that enables TDM to IP transition.