PSTN

What it is

The PSTN is the network of national carriers, mobile operators and interconnects that lets any phone number call any other. Even when a call spends most of its journey as internet packets, it enters and leaves through carriers that follow PSTN conventions: E.164 numbering, caller ID, ring-back tones and, most consequentially for a voice agent, narrow-band audio. Traditional telephone audio is sampled at 8 kilohertz and compressed with codecs designed for intelligibility between humans, not for machine recognition.

Mobile networks and some VoIP paths carry wider-band audio, but any call that crosses a legacy interconnect is squeezed back to narrow-band.

Why it matters when buying

Speech recognition, end-of-turn detection and voice quality all degrade on telephone audio compared with the browser demos vendors like to show. Consonants that distinguish "F" from "S" fall above the frequencies the network carries, which is one reason spelled names and alphanumeric codes are hard on the phone. Carrier-side processing (noise suppression, comfort noise, echo cancellation) can also interfere with the agent's own detectors. Any accuracy or latency figure you are shown should be asked about with "over a real phone call?" attached.

What to ask

Ask for every demo and every benchmark to be run over a real phone call from your country, not from a browser. Ask which codecs the platform negotiates and whether it prefers wideband when the carrier offers it. Ask how the agent handles alphanumeric codes and spelled names on narrow-band audio. Ask about regional interconnect: where does a call from your market enter the vendor's platform?