Is it necessary for a court order to gain access of the content of text messages sent on a phone number on a master phone account?

In this case, my wife's cell phone is an "extra line" on my account. I have reason to believe there is infidelity (despite her denials) and would like to obtain the content of these texts as further evidence. The phone company said no, that it was a privacy issue despite the account falling under my name.

Can I push the issue with the phone company without raising any flags to my wife that a court order may cause? Unfortuately, I have not been able to install a key logger on her phone as I have been able to on our shared computer.

Thanks.