This is sweet.. the FBI have been hackin the darn system for years !!
"the surveillance system, called DCSNet, for Digital Collection System Network, connects FBI wiretapping rooms to switches controlled by traditional land-line operator and internet-telephony providers and cellular companies. It is far more intricately woven into the nation's telecom infrastructure than observers suspected." - [ Source ]
Many of the details of the system and its full capabilities were redacted from these documents