You can read about the mechanics of telephone taps in this Network World story.
You can read about the role carriers may or may not play in carrying out warrantless phone taps in this accompanying story.
What you're not going to be able to read about -- apparently ever -- is a Senate investigation of whether the Bush Administration has been acting illegally in spying on U.S. citizens without a warrant, as most legal experts and this blogger certainly believe. This Washington Post story explains how that investigation, which not long ago had strong bipartisan support, fizzled before it ever got underway.
So much for checks and balances.