I am not sure how well this would work, but you could take your voice source, lowpass filter it at 3 KHz, and then run it into a 1-bit ADC at a 6 KHz clock rate. You would use that 1 bit to modulate the PLL digital divsor using the "data" pin on that chip. You would need the PLL loop bandwidth to be perhaps 20 KHz, which is easily do-able.
At the other end, you would demodlate the signal as a 0 or 1, and then lowpass filter it again with a 3 KHz filter to turn it back to an analog signal, amplify, and there you go!
1 bit analog to digital converters were popular a decade ago....but they still live on in effect in class D amplifers, etc, which are "1-bit" analog devices.