The Free On-line Dictionary of Computing (19 January 2023):

Post, Telephone and Telegraph administration
PTT

    (PTT) One of the many national
   bodies responsible for providing communications services in a
   particular country.  Traditionally, PTTs had monopolies in
   their respective countries.  This monopoly was first broken in
   the USA, with the UK joining somewhat later.  Currently the
   markets are being deregulated in Europe as well as other parts
   of the world.

   Well-known PTTs include MCI, AT&T, and British Telecom.

   Compare: telco.

   (1998-05-18)