From EFF.org
Electronic Eavesdropping is Legally Hard for the Government, But Technically Easy
As you learned in the last section, wiretapping is legally difficult for the government: it must obtain a hard-to-get intercept order or "super-warrant" from a court, subject to strict oversight and variety of strong privacy protections. However, wiretapping is typically very technically easy for the government. For example, practically anyone within range of your laptop's wireless signal, including the government, can intercept your wireless Internet communications. Similarly, practically anyone within range of your cell phone's radio signal, including the government, can — with a few hundred bucks to buy the right equipment — eavesdrop on your cell phone conversations.
As far as communications that travel over telecommunications' companies cables and wires rather than (or in addition to) traveling over the air, the government has very sophisticated wiretapping capabilities. For example, using a nationwide surveillance system called "DCSNet" ("DCS" stands for "Digital Collection System") that is tied into key telecommunications switches across the country, FBI agents can from the comfort of their field offices "go up" on a particular phone line and start intercepting or pen-trap tapping wireline phone calls, cellular phone calls, SMS text messages and push-to-talk communications, or start tracking a cell phone's location, at a moment's notice. The government is believed to have similar capabilities when it comes to Internet communications. The extensive and powerful capabilities of the DCSNet, first uncovered in government documents that EFF obtained in a Freedom of Information Act lawsuit (details at
http://www.eff.org/issues/foia/061708CKK), are well-summarized in the Wired.com article
"Point, Click...Eavesdrop: How the FBI Wiretap Net Operates".