[1] This enables the mode 3 device to control the line, and gain priority in need.
It is a common alternative to parallel connection.
For example, a dial-up computer modem is generally provided with two line connectors, often labelled line and phone.
This prevents accidental interruption of the data service by raising the handset, which could occur if the two devices were simply connected in parallel.
This article related to telephony is a stub.