A. First decide what your goals are in this marriage.
Do you want to have a happy and trusting relationship with her? Do you believe that she wants to be faithful to you? If so, hiring a professional stalker won't make that happen.
Or are you just trying to prove your suspicions are true so you'll stand a better chance in divorce court?
It's evident that though you have taken her back since the affair, you're both on edge emotionally.
What if you discover that the phone number belongs to no one of significance? And what if you did discover that she is having an affair? Would you then chose divorce? Counseling? Revenge?
If you want your marriage to be healed and rebuilt, start with the problems that you already can see, get the counseling you need and work from there.
If she is being dishonest with you, you may never know all the details, but a lack of integrity is too hard to hide. If it's time to call it quits, you'll know, even without the help of a detective.
But if you and she can get to the root of this and change your marriage, only openness and humility can get you on that path.