Usually "reasonable" is decided by the jury. But when it comes to privacy, the government defines the reasonable expectation of privacy. Not many people know that SCOTUS first wiretapping decision concluded there is no reasonable expectation of privacy in any telephone call.
It is a mistake to think that the government will recognize any privacy that you don't deliberately and carefully create and guard.
That particular decision came about in a time where there were banks of human operators manually switching calls where they needed to go and they needed to be able to hear the caller and recipient to route the calls. Technically, that was a correct decision for the day as you *had* to have a third party listening in just to get many calls accomplished.