In some old party-line systems, each phone would be assigned a different ringing frequency. Filters in the drop would be used to eliminate ringing voltage of other phones on the party line. Higher voltages aren't required because the telephone does not have to be rung from a long distance. This means the voltage drop in the twisted pair may remain low. In a cable telephony system, the NID at the home detects the off-hook condition, then signals its switch.
Conversations take place in the off-hook condition, as does dialling.
A total of 16 numbers and symbols may be represented by the tones, of which 12 are normally available on consumer telephone instruments. At the NID, tones are detected using a tone decoder, which supplies digit information to the switch. A number of tones are used to communicate a call's progress. We will discuss only the ones of most common interest. The dial tone usually consists of two tones of equal amplitude.