It really is the accepted industry-standard term. Even an order taker at the telephone company's business office will immediately recognize this term.
The reason that many equipment manufacturers require a POTS line is to avoid the need to dial additional digits in order to get an outside line (9 for example). Also, line voltages vary greatly on various PBX or key systems. The manufacturers want to see uniform voltages which are supposed to be present on POTS lines. They rarely are any more.
"But the guy at Home Depot said it would work."