Dial Tone (DT). This is a 33 c/s continuous note and is applied to the line after the subscriber has lifted his handset and the switching equipment
has allocated him an available outlet for this call to proceed. There would have been a physical limit on the number of calls an exchange could handle
so if all equipment was already in use, the subscriber would not get dial tone. The actual pitch of the dial Tone varied from exchange to exchange
depending on the adjustment of the ring generator.
Busy Tone (BT). A higher pitched note of 400 c/s interrupted to give a cadence of seconds on, seconds off. Busy tone
indicated either that the called subscriber is already off-hook (busy) or that the route to the called subscriber is congested. In later systems,
a slightly different cadence was introduced in order to distinguish between these two scenarios.
Number Unobtainable Tone (NUT) . Identical pitch to the busy tone but continuous. This tone is used to indicate that a number is out
of service, faulty or that a spare line has been dialled.
Ring Tone (RT). A tone of 133c/s which interrupted in the same cadence as the ring current which rings the telephone's bell
at the called party's end : seconds on, seconds off.