V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

POTS
       Plain Old Telephone System (slang, Usenet, IRC)


The Free On-line Dictionary of Computing (30 December 2018):

Plain Old Telephone Service
POTS

    (POTS) The traditional voice service provided
   by phone companies, especially when opposed to data services.

   Note that the acronym POTS is sometimes expanded as "Plain Old
   Telephone System" in which sense it is synonymous to Public
   Switched Telephone Network but used somewhat derogatively.

   (1998-05-18)