Alright, I can grant that is true, although if you assume an ISP like Comcast has thousands (or more) simultaneous Skype calls, and they come and go often enough that it's likely you'll be dealing with many starting at close to the same time... there's a lot to track there. Also, bear in mind, you have to weed this out from all the unidirectional calls: Comcast to [other ISP] and [other ISP to Comcast] calls, so you can't even make one-to-one matches reliably.

And of course, when you compare to the tech industry alternative: Microsoft knows who is talking to who straight up (and on a global scale), whereas the ISP can only guess. And while the ISP's understanding of who is calling who ends at the household, Microsoft knows which user accounts made the call, which is much more likely to correlate to individual people.

MS has access to the payload as well.