If you think you can handle it, don't tell her you know about her cellphone yet. Do you have detailed billing on the phone? If so, collect those bills and store them in a safe place (hopefully they're available online; if so, print them to .PDF and save them by some nondescriptive name, and shoot a copy of one to a web-based e-mail address like yahoo or g-mail). As SW said, you want to gather as much info as possible right now before you confront her.
Who is this other guy? Is he married?
I have been where you are, as has SW, as has dozens of other people on this forum alone. Although you've just taken a BODY BLOW, there is some "good" news in that at least you now have a reason why your wife is behaving the way she is. There's also a good chance that much of what she is telling you about her lack of feelings toward you and the marriage is nothing more than affair-script "re-writing of marital history."
