In that particular pvoc implementation (Mark Dolson original author decades ago), the first frame in an analysis file is all-zeroes. Other implementations may skip that, but the arithmetic is the same. In the general case, all bin phases have to start from somewhere on the circle, so to speak, and the default choice is zero, just as it tends to be for any digital oscillator. since pvoc is based on the FFT, converting phase increments to frequency, each bin has a (default) centre frequency arranged linearly from DC to Nyquist.
In theory, any other starting phase could be used; one would expect this to be the same value for all bins, but there have been suggestions to start with randomised phases. Probably less useful for pvoc in analysis mode, but often a way to avoid extreme peak amplitude values when creating an oscillator bank. The Dolson pvoc code is certainly more difficult and opaque to read than others (some clever code involved to apply overlapped windowing while avoiding all memory movements); but the payoff is that it is significantly faster than other more "straight-forward" implementations.
