How does a standard telephone POTS line work?

The main principle at work is impedance matching, although the hybrid must also block the 48 V direct current of the POTS telephone line from the normal audio side. Some inexpensive designs connect to the handset cord, with a button to activate either the handset or the hybrid. These only cost around US$100, need no power, and don't even need to block DC as none goes through the handset. More expensive versions can cost thousands of dollars or more, but can handle multiple lines, and may connect to a computer so that a producer can keep up with who is on which line, and what goes on the air next. Many also perform audio level compression, and even full digital signal processing to make the bandwidth-limited telephone audio sound more acceptable on-air.