Good answer from Chris (+5)
Yes, it is all very vague. Phone systems themselves tend not to fall under PCI per se, however where most customers get caught out is with ancillary applications such as call recording.
Again, the PCI guidelines for call recording are vague and basically state that you are not allowed to store CVV/CV2 numbers, unless you're unable not to store them. Very helpful.
My experience with PCI & call recording in particular is not to store the CVV/CV2 numbers under any circumstances. There are a number of solutions available to assist with this, but are dependant on the call recording platform being used.
Generally SRTP can't be used with call recording solutions. If you do try it, you'll find the recorded conversations are very secure, although slightly difficult to replay