If NSA agents, or agents of any other government agency, approach an internet service provider or phone company and ask for information on an individual, the transfer is "voluntary" in the same way that a shopkeeper "voluntarily" pays protection money to the mobster who says "Nice store you got. Wouldn't want to see anything bad happen to it, would we now?"
The private individuals or businesses know that the Feds will find ways to make their lives miserable if they don't "voluntarily" fork over the information in question.