Line isolation transformer

This is a special type of transformer that is used to couple [telephone] equipment to [electronic] equipment. It is made with a primary winding that is designed to have a [DC] voltage impressed upon it. It is also made with a special type of magnetic core that is not seen in usual [Transformers}. It has a specific 600 Ohm input impedance which matches the telephone equipment. The output of some of these transformers are at 1K Ohms and at line level.