1) Communication circuit 
A tie line, also known as a tie trunk, is a communication connection between extensions of a private telephone system, typically two PBXes. ...
A private circuit that connects two ACDs or PBXs across a wide area.
a telephone circuit permanently connecting two private telephone installations
A horizontal line constructed across a two phase region of a binary phase diagram; its intersections with the phase boundaries on either end represent the equilibrium conditions of the respective phases at the temperature in question.