I'd present her with a list, turned 180 degrees around (behaviours that YOU are engaging in, and SHE is the one looking at it from the outside.) Ask her what SHE'D make of it?
1.) Together at work all day, yet texting constantly when apart.
2.) Panicky shutdown of phone when surprised by her entrance.
3.) Sudden increased interest in sex, both in initiating it and in enjoyment of it far more than is historically so.
4.) Non-drinker, yet beer is missing from the fridge.
5.) "What we have works, so I'm just gonna hang in there." What in hell have you been thinking about? Hang in there as opposed to... what? And why?
6.) Sudden increased interest in appearance and fitness.
Google "signs of infidelity" or "signs of cheating" It reads like this list. She's either cheating already, or is about a one-ounce trigger-pull from doing so. The sudden interest in appearance, the sudden increased interest and frequency of sex (especially with all-new position(s) are DEFCON ONE red flag signals, always. One hell of a string of co-incidences, if you ask me. And also... most folks just get a cellphone with a contract... not many take the trouble to do the tracfone thing, unless money is a big priority. This is an excellent method of 'covering your tracks,' as well; has she always had this setup, or was that changed sometime around this developing interest? She seems quite savvy in the 'cover your tracks' department, and people with nothing to hide, hide nothing (to quote Dr. Phil... can't stand the guy, but he's got some useful one-liners.)
Turn the tables on her, she's the faithful one and seeing all this crap in YOUR behavior, and ask her what SHE would think... and if she gives you the "nothing, honey, you know I trust you...," then add liar-to-your-face to the list as well. She'd be flying at you with rockets in her fanny if YOU were the perpetrator of all this (unless she doesn't care, or is of that extreme minority who actually gets off on such things.) And finally... "Luke, search your feelings!" Perhaps YOU are one of those who could get off on the idea of your wife with another guy, provided you got to share in the experience, or watch footage when she returns home, with blow-by-blow commentary. Really works for some folks (that would be me, and others, but different strokes....) Don't let it just lie... you deserve to be reassured, and dealt with truthfully, at a minimum. If she truly loves you, and she sees your distress, she will take steps to be far more transparent or alter her behavior. She should be flattered that you find her attractive enough, and hot enough, that you are concerned by these things... it's an indicator of how much you care for her, and your partnership (it's also a symptom of basic male insecurity surrounding sexual matters, but that's hardwired and not really subject to modification.) I'll send light your way, and all my wishes for a favorable resolution, one you both can live with!