Basically you need an oscillator, whose frequecy is controllable. That is what changes the tone, the frequency of the signal. Then you need a powerful amplifier and that is the siren. sometimes, they mix two signals, to obtain different tones.
You can use digital counter with low pass filter or to sum 2 or more sine waves to generate more then 2 tones
I guess that in commercial security system the microprocessor used to generate the tones
For 24VDC you will need voltage regulator for COMS levels the siren driver can works with 24V
Use the DIP switches to switch in and out of the circuit some capacitors in parallel with the timing caps, to change the frequency. Alternatively, you can use the switches to add resistance in parallel or in series with the timing resistors.
In the end, what matters is that you change the frequency.