Y'll need to understand the difference between a "pen register" and "listening in on your phone calls".
A pen register records the numbers called from a phone line. It does not record the conversations. All this "metadata" you have been hearing about is like placing a pen register on every phone in America. Then the NSA sees what phones are talking to each other. Those phones which call suspected terrorists' phone numbers then pop up on the NSA's radar for further examination.
In order to actually listen to your phone conversations, the Supreme Court determined in
Katz v. United States that the cops have to get a warrant.
To place a pen register on your phone, the cops do NOT have to get a warrant, according to the Supreme Court's
Smith v. Maryland decision.
In 1986, Congress passed the
Electronics Privacy Communication Act which requires law enforcement to get an
ex parte order from a judge to place a pen register on a phone.
With the advent of the internet, law enforcement wanted to be able to do with people's internet communications what they were doing with phone pen registers. This was added to a long wish list. When 9/11 happened, law enforcement (not letting a good crisis go to waste) added all their wish list items into what became the USA Patriot Act of 2001.
If you read
Section 216 of the Patriot Act, you find "pen registers" are extended to electronic communication, such as the internet.
As I said, this was part of a long list of expanded powers desired by law enforcment. It is important to understand they wanted these extra powers long
before 9/11.
The significance of that is that these tools are not just intended for the War on Terra™. These are intended for
all future law enforcement activity.
Politicians who attempt to assuage you that this is just for catching terrorists are either incredibly stupid, or evil.
And remember, kids, there's a reason we have a Fourth Amendment. Our founders knew firsthand that tyrants have this really nasty habit of finding what they are looking for. So the "if you aren't doing anything wrong, you have nothing to worry about" pacifier for the rubes is completely bogus.