Are you quite sure they aren't collecting content? The storage is feasible, and several remarks (slips?) by people in a position to know suggest that they are in fact doing this, if not to everyone, then to hundreds of thousands or even millions of people.
> Are you quite sure they aren't collecting content?
It seems to depend entirely on the program in question, and the legal authorities for each program.
For foreign intelligence actually collected overseas under EO 12333 they have programs that collect content (e.g. SMS), which have existed in one form or another since the beginnings of the Cold War.
For foreign intelligence conducted on U.S. soil they have to fall within the boundaries of Fourth Amendment reasonable search (and related legal rulings like Smith v. Maryland) so they wouldn't capture content. But on the other hand they can still capture content in targeted fashion against non-U.S. persons in certain scenarios, and that content might itself involve conversations involving a U.S. person and still be a legal search.
It seems to depend entirely on the program in question
Accessibility does, but it has nothing to do with the agency. "Does the government store the content of US calls?" can be answered in the aggregate without reference to specific programs. If the content is there, an agency only has to acquire the authorities required to access what is already there.
So, the question remains: is the content there to access, given proper authorities?
There's multiple levels of authorities though. Collecting the calls at all would itself require some legal authority. Searching the calls you collected would then need yet another legal authority.
http://blog.rubbingalcoholic.com/post/52913031241/its-not-ju...
https://www.schneier.com/blog/archives/2013/06/evidence_that...