Plain old telephone service (POTS), or plain ordinary telephone system, is a retronym for voice-grade telephone service employing analog signal transmission over copper loops. POTS was the standard service offering from telephone companies from 1876 until 1988 in the United States when the Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) was introduced, followed by cellular telephone systems, and voice over IP (VoIP).^[[Plain old telephone service - Wikipedia](https://en.wikipedia.org/wiki/Plain_old_telephone_service)] The expression “POTS” was initially intended as a joke but is now used as a common expression in the telecommunications industry. ^[[What are FXS and FXO •• Analog ports | 3CX](https://www.3cx.com/pbx/fxs-fxo/)]