When it comes to modern law enforcement surveillance, no one watches the wiretappers.

The vast majority of law enforcement’s demands that phone carriers and Internet services hand over users’ private data don’t require a warrant, and occur with little or no accountability. It’s not just that we don’t know how much surveillance takes place. To paraphrase Donald Rumsfeld, we don’t even know what we don’t know about how much the government knows about us.

But we’re finally starting to learn more. On Monday, Congressman Ed Markey released a collection of letters he received from major phone carriers in response to a query he sent them earlier this year, demanding that they reveal how often they give users’ data to the government and under what circumstances. The responses were patchy, evasive, and in many places leave out key information. But I’ve assembled a few bullet points from them that capture the massive surveillance affecting phone users.

• Sprint received 500,000 subpoenas for its data from law enforcement in the last year. That doesn’t include court orders for wiretaps and location data, which Sprint didn’t track annually but which added up to 325,982 requests in the last five years. The company also says it doesn’t have the resources to track how many of those requests it responded to or rejected. The company has 221 employees dedicated to processing and responding to government requests for its data.
• Verizon received 260,000 requests for its users data in 2011, including wiretaps, calling records, text message information, and location information, but doesn’t add how many were fulfilled.
• AT&T received 131,400 subpoenas in criminal cases for its information in 2011, as well as 49,700 warrants or orders that it hand over data. It rejected 965 of them. The company says it employees more than 100 staffers full-time to respond to law enforcement demands.
• T-Mobile told Congressman Markey it “does not disclose” the number of law enforcement requests it receives or complies with.
• MetroPCS says it received fewer than 12,000 requests a month on average for the last six years.
• Cricket received 42,000 requests last year, and U.S. Cellular received 19,734 requests in 2011.
• The New York Times counts a total of 1.3 million requests for users’ information in the last year based on Markey’s data.
• The number of data requests seems to be growing quickly across the board. The major carriers who measured the growth in requests over time agreed that law enforcement demands have risen 12-16% year-over-year.

Markey’s request for this information followed a groundbreaking report by the American Civil Liberties Union based on Freedom of Information Act requests to police departments around the country for evidence of their policies on data requests to phone carriers. The results showed a disturbing rise in the use of phones as a central tool for law enforcement, including tracking location and even using “tower dumps”–collections of all the stored information collected from all users of a cell phone tower–without a warrant.

It’s important to remember that the information revealed Monday includes “tower dumps,” too, says Chris Calebrese, an attorney with the ACLU. “Just the sheer volume of orders is amazing, but a significant chunk are dumps from entire cell towers,” he says. “That means tons of people’s information is being grabbed with a single one of these orders.”

The numbers also bring home the fact that the official wiretap report released earlier this month is now all but useless. That report showed a 14% drop in wiretaps. But with less than 3,000 of those real-time communication requests reported, they represent less than a third of a percent of actual surveillance requests.

Markey’s surveillance report puts in perspective the privacy issues for Internet services compared with phone companies. Google, which releases a bi-annual report on law enforcement requests, received only 12,271 requests for its information in 2011, compared with the hundreds of thousands received by phone companies.

The ACLU’s Calabrese says Markey’s data points to the need for both more transparency and laws to better regulate on companies hand over users’ data. He points to the GPS Act making its way through Congress, which would require a warrant to track a user’s location with a cell phone. “It’s amazing that Congress has never regulated what standard law enforcement should use before tracking someone’s location. The appropriate standard is a probable cause search warrant,” he says. ” Clearly the cell phone has become the central tool for law enforcement investigations, even as the laws governing what information about them can access accessed and with what standard have become entirely out of date.”

The full responses to Markey’s letter are posted here.