tie line

noun Telecommunications.
  1. a line that connects two or more extensions in a PBX telephone system.
  2. a private telephone channel, leased from a telephone company, that connects two or more PBX systems.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for tie line

tie line

  1. a telephone line between two private branch exchanges or private exchanges that may or may not pass through a main exchange
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012