Two choices. #2 is better than #1, but you have to have the stomach for it.
#1. You confront her now, and say "Look, I know all about you and _______ (OM), and it needs to stop. This is incredibly disrespectful to me, to our marriage, and to our family."
Ordinarily, this will work, because a cheating spouse won't know what you know, and so long as you refuse to reveal your intel, and you're a good bluffer, they think "Oh chit -- he knows about my hook-up at the hotel, or he knows about my STD test result," or whatever. Even tho all you got is the cellphone activity (and not even its content). But since your wife already has been flagged on the cellphone account, you're just going to get a "Oh, he's just a friend," which is a pretty cheeseless tunnel, in my experience. More on that if you choose to go this route.
#2. GATHER MORE INTEL. Keylogger on her computer, voice-activated recorder under the front seat of her car, hire a PI (preferred, but expensive). It WILL yield fruit, but you're going to have to be patient because they're probably going to be real careful for the next few weeks, since you asked about the cellphone bill.
The other thing you could do would be to pull of the world's best acting job, like nothing is bothering you, and then wait for her to leave her phone lying around, unlocked, so you can see the CONTENT of text messages (I'm guessing her TMs are thru the roof???).
If it were me, I'd act as if nothing were bothering me, so she'll get careless and think she's in the clear, and I'd use the next month to gather as much intel as I could, and then I would CONFRONT and EXPOSE.
