POTS

POTS is the public switched telephone network that carried voice calls over copper twisted-pair wires from the 1880s onward. It uses circuit switching: when you dial, the telephone exchange physically connects your line to the destination line for the duration of the call, dedicating bandwidth to that single connection. The network operates at audio frequencies between roughly 300 Hz and 3.4 kHz, which is why POTS calls sound tinny compared to modern codecs.

The system relies on loop current: a voltage typically between 24 and 48 volts DC maintained by the telephone exchange through the copper pair. Off-hook signals (picking up the handset) complete a circuit that draws current; dial pulses or DTMF tones transmit the called number. This simplicity meant POTS equipment needed minimal power and worked reliably over poor-quality lines and long distances.

POTS defines a specific line impedance of 600 ohms and standardized signaling protocols that made any telephone compatible with any exchange worldwide. The network carries direct current for signaling, alternating current for the voice signal, and 90-volt AC pulses for the ringer. Modern devices called analog telephone adapters (ATAs) convert VoIP to POTS signals so legacy phones continue working on internet-based services.

Decline and persistence

Carriers have decommissioned POTS infrastructure in many regions since 2000, replacing it with digital systems and VoIP. However, POTS still provides uninterrupted service during power outages (the exchange supplies loop current) and requires no configuration, making it valuable for alarm systems, fax machines, and rural areas where broadband is unavailable. Some jurisdictions legally mandate POTS availability or equivalent service for emergency access.

The term "plain old" was deliberate: it emphasized reliability and ubiquity against flashy new services in the 1990s. Today it marks a technology that is simultaneously obsolete in wealthy markets and irreplaceable where infrastructure is sparse.