This circuit generates the busy tone of ordinary telephones. The transistors can be the BC548 and the components of the T filter and the multivibrator can be changed to obtain the exact tone.
The circuit of the figure can provide voltages of 5, 8.9 and 12.5 V at its output, all doing this...
In the figure we have an application of ON Semiconductor's own semiconductor manual from the 80's....
This circuit was found in Philips documentation from 1969. The components are from the time...