With an analogue telephone line, the information about the dial-up connection, as well as the voice information, are transferred down telephone line in parallel, so special dialling procedures are used. The older method is so-called pulse dialling, which dates from the time of switching centres with rotary dials. The individually transmitted pulses turn the dial into a certain position that ultimately establishes the connection. The pause between the pulses ensures the correct control of the various rotary dials.
Current analogue telephone lines use the system known as multi-frequency dialling (DTMF). Each key tone produces two defined frequencies (DTMF - Dual Tone Multi Frequency), which initiate the dialling process at the telephone switching station.