So if I call you from outside 123 Main Street, Anytown on Tuesday at 6:37pm, what are we talking about? Even if one of us is known criminal or terrorist, the fact that conversation took place is merely suspicious rather than probative.
As it happens, I'm from Europe, and one thing I do miss about the EU is that citizens of that territory enjoy a explicit and robust constitutional right to privacy that does not exist in the US, not to mention a unilateral right of access to and control over their personal data in both governmental and commercial spheres. I have called on many occasions for an amendment to the US Constitution that would confer such rights on Americans instead of relying on the vague and implicit 'discovery' of a privacy right that did not exist before 1965 and which could be taken away as easily as it was given.