I think there's a difference between someone invading someone else's privacy and invading someone else's secrecy, don't you? I think that when one partner violates the privacy of the marital relationship by going outside of it to have their needs met, I don't see a problem with the innocent spouse needing to check that this violation is not continuing. Of course, these opinions are at odds with the opinions of a wandering spouse, but let me tell you, a spouse with nothing to hide, has nothing to hide, and a spouse who is remorseful and wishes to make amends will not balk at having their movements checked and will not be at odds with my opinion. But snooping , while it may uncover truths, is a terrible way to live, living with someone you can't trust. I did that for a while.
Looking at cell phone bills, I can tell you from experience, if you thought highly enough of yourself so as to marry a fairly intelligent mate, your intelligent mate will be clever enough not to use the cell phone for outgoing calls. Mine bought an ATT 800 card, and called from phone booths. The lesson I learned from that was, they can find ways to outwit you.
