Binary

POTS: Plain old telephone service. ( 64Kbit voice )

As opposed to PANS : Pretty advanced network services.( HD voice plus a plethora of advanced facilities sometimes including video.)

PSTN is built around POTS but has had many added facilities since 1984, ( CLI, Call diversion, Ringback, Blacklist, ISDN etc)

BT's Digital voice appears to be a voice only 64Kbit service so still POTs but could run PANs services using the same platform and different end codecs ( Defaulting to 64K during Codec negotiation at call setup if different end codecs are in place.)

In reply to a post by binary:

Out of curiosity, tow are you defining POTS, and how is your definition distinct from PSTN?

POTS is hardly an official, technical description!

The terms are often use interchangeably, but technically have different meanings.

POTS (Plain Old Telephone Service) is an analogue telephone service over copper wires.

PSTN (Public Switched Telephone Network) is the overall switched network, and is not only connected over copper wires. For example BT used fibre (TPON) to connect some areas to the PSTN.