[Sir Napsalot]

To answer myself:

"The phone records don't include the substance of the calls — they're just a written tally of who called whom and how long the calls lasted." (Source: NPR)

[HollyB]

Perhaps, however if the Justice dept was concerned about a leak, it may have been tapping as mentioned in this transcript.

http://www.hughhewitt.com/congressman-devin-nunes-previews-ways-and-means-hearing-friday-plus-a-bombshell-on-the-ap-scandal/

“DN: No, I absolutely do not, especially after this wiretapping incident, essentially, of the House of Representative. I don’t think people are focusing on the right thing when they talk about going after the AP reporters. The big problem that I see is that they actually tapped right where I’m sitting right now, the Cloak Room.”

[SpaceBar]

a written tally of who called whom and how long the calls lasted...

That's called traffic analysis. It was a mainstay of World War II allied intelligence, even when the specific encryption used couldn't be broken. A lot can be inferred just by knowing who's in communication with who.