I second modeltrainparts suggestion of using EZ Line. It looks realistic, is easy to use, and has an abundance of give/flexibility ... so when the line is accidentally bumped by hand or other object EZ Line flexes, not pulling telephone poles down.
I used to use sewing thread, however, occasionally a visitor would point at something on the layout their hand would hit the sewing thread which was taught between the poles. Because of the tension on the thread several telephone poles would go down and that was alway a mess. EZLine solved this problem AND it looks much better than sewing thread IMHO.