You need 96 VAC at 25 Hz.
You can take a 50 Watt audio amp, and connect an audio test generator up to it as a source oscillator in series with a push button switch.
You can get a 120 Volt to 12 Volt power transformer, rated to at least 3 amps, and connect it up as a step up transformer on the output of the audio amplifier. Across the output of the amplifier in parallel to the transformer, put a 27 ohm 20 Watt resistor across the output terminals to give the amplifier a resistive termination load.
Before connecting up the phones, put a true RMS DVM across the secondary of the 120 Volt side of the transformer that is going to drive the phones. Set the audio generator for 25 Hz, with a level that is adequate to drive the input of the amplifier. Send the signal to the amplifier. On the output side, adjust the volume control so that your DVM reads 96 to 98 VAC.
Now you can connect the phones as you wish. Your phones should ring just as if they were connected to a telephone line.