A butt set, also known as a goat is a test set for telephone lines commonly used by technicians in telephone exchanges and outside plant. There are many different butt sets, but all of them share a few things in common:

• A rotary dial (for older models) or touch tone keypad in order to dial telephone numbers.
• Receiver
• Mouthpiece
• Talk/Monitor toggle switch
• Test leads, such as alligator clips, heat coil adapter or pins, to interface with the telephone system.
• A removable hook for placing the butt set on a tool belt. It can be moved to the receiver side, mouthpiece side, or removed altogether based on the technician's preference

Newer models, such as the Harris TS44 have a speaker function, mute, volume control, number recall, and polarity indicator.

Wherever there is an apperance, whether in the exchange or field, a butt set can be used to "butt in" to a telephone circuit using the test leads. The most common way is to clip on with alligator clips to metallic lugs, either on the frame or at a cross box , on the tip and ring side of a phone line. This allows the technician to check for dial tone, ANAC a number to determine the phone line being worked on, or place a call.

When using a butt set, the technician is always mindful that he is essentially an extension of the customer's home phone. If the Talk/Monitor toggle is moved from monitor to talk while the customer is on the line, it will sound as if someone else has picked up the phone. Also, a technician can be interrupted by a confused customer, who hears strange voices on his line.