A butt set is a specialized telephone handset that is used by telephone lineman.  This handset is used to test telephone lines, by the technicians, to make sure that they are operating properly.

The features of a talk and monitor butt set are an amplified speaker for both monitor and talk modes, the inclusion of a 9V battery, 12 memory positions, a lightweight and compact design, a last number redial function, the profile of the design allows the handset to be held between the shoulder and the ear, easily, moisture and dust-resistant housing, and last number redial function.

The butt set connects by using a pair of test leads.  The use of these sets allow lineman to answer calls, make calls, and monitor calls.