Tony and Sage are also giving you decent advice. Their perspectives may differ from mine (and yours) because all of our situations were different. You will have to make the decisions, right or wrong, about your relationship with your W. In my situation, the best choice was balance between the need to have her eliminate the OM (and yes, snooping was involved) and working on myself. Are there good reasons to snoop? Of course, but keep in mind that snooping may drive your W away. In my case it worked well, and for me, snooping after d-day allowed me to regain some of the lost trust. I'm nowhere near "recovered", but we are doing much better.
Ultimatums are best handled with care. Michele has much to say about them in her DR book. In my own case, the ultimatum was given after my W failed to keep her promises regarding contact with OM#3 several times over, and my own MC recommended I make a clear stand. The problem with ultimatums is that you have to be willing to follow through. For me, I had come to the point of knowing that I was doing all I could do for the marriage without my W's committment (several months after d-day). I decided that if she wasn't going to help, then the marriage wasn't worth saving. The ultimatum was my last resort, and it has worked so far (I'll reevaluate that statement in a year or so).
For me, a combination of recovery programs and ideas seems to have worked best, some from DR/DB and some from MB. In my own situation, my W and I decided within a few months of d-day to consolidate our separate lives. Before, we had separate bank accounts, credit cards, bills, etc. In addition, we both had numerous email accounts with "secret" passwords. Now we have joint accounts and share all login and password information with each other. That makes it much harder for an affair to flourish - not impossible, just more difficult. In your situation, the cell phone billing should eventually become an issue for discussion, but you may want to save that discussion for a time when you feel your W is committed to the marriage. Otherwise, it will be one more thing that will drive her away.
Ultimately, it comes down to the question of whether you want your W to stay or leave. After you answer that question (and I understand you might feel ambivalent at times), then figure out which things you need to do to make that happen.
