Reuters recently announced the DEA, FBI, IRS, CIA, and NSA have indeed been listening in on phone calls. Those agencies have formed the Special Operations Division ("SOD") and have in fact been listening in on phone calls without a warrant or even reasonable suspicion for non-terrorist activity. The SOD claims that this activity is perfectly legal and has been going on for decades.
What is most disturbing about this practice, is that the SOD creates "parallel stories" when they present their information to judges or prosecutors. This means that they lie to the judges and prosecutors about how they obtained the information - even to other branches of government. Instead of telling the judges or prosecutors how they got the information, they make up another story, that they call the "parallel story", which is a legal way of obtaining this information. Most of the information the SOD has illegally gathered has been used to target drug dealers - not drug dealers with terrorist connections - just drug dealers.
First, there is the issue of Brady violations - information that goes to prove the defendant is not guilty, which the prosecutor must disclose. Since the prosecutor may not even be aware of this information, this poses a huge problem.
Second, if the agents testify under oath about how they obtained the information - that is perjury. To date, perjury is illegal and a very big deal.