[stray]

The tone was a continuous 350 Hz and 440 Hz sine wave combination, and was generated at the Central Office (likely by a Western Electric No. 5 ESS). It started when the telephone went off-hook and stopped when you started dialing a number.

Dialing a number was done by DTMF dual-tone, multi-frequency tones or by pulses (interruptions of the circuit).

And to answer your likely next question: No. The ringing tone was not synchronized with the bell on the target telephone.