To use a PLL for this sort of application needs one that has a lock range that can track across both the high tone and the low tone frequencies. The VCO control voltage from the PLL's phase comparator is used by a separate comparator circuit which is set up to convert it into logic levels. When the PLL detects a low tone the comparator flips to the "0" state, or flips to a "1" when the PLL detects the high state.
But why not use a special PLL designed for the job where it's all in one package? The Exar corporation have made just such a PLL demodulation device for some years(XR2212). For FSK modulation also see the FSK generator (XR2206).
I have used these in several low cost modem designs which were approved by British Telecom for use on PSTN lines. Note that most PLL devices still require band pass filters to prevent false decoding on some of the harmonic tones that you can get on PSTN lines.
