A butt-set is used to test the telephone line at selective points such as the telco NID box or where the line connects to the payphone. The butt-set can also be used to monitor a phones activity on the line such as dial outpulsing.

- Integrated circuit design.
- Tone (DTMF) or pulse (Rotary).
- Line powered for talk, monitor or dialing function.
- Built-in ringer.
- Built-in polarity guard.
- Last number redial.
- Mute button. This button operates only as long as user keeps it depressed.
- Includes polarity test. LED lights up green when polarity is correct.
- Talk/Mon (monitor) switch. (MON position allows line monitoring without disruption)