WordNet (r) 3.0 (2006):

telco
    n 1: a public utility that provides telephone service [syn:
         telephone company, telephone service, phone company,
         phone service, telco]

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

telco

    (from telephone company) A company
   providing phone services to end users.  The company may or may
   not provide other phone services such operating
   long-distance/international backbones but the name telco
   usually emphasises its operation as a local service provider.

   Compare: PTT.

   (1998-05-18)