Cell phone question

[mathnerd]

I have a question for the cell phone gurus out there. Let's say a person has:

-A smart phone from the fruity company
-Service though the company who's name is three letter and a symbol
-The phone is actually an additional line on another person's account.
-The phone is password locked and always in the owner's possession

Assuming there is no court order in place, how much data can the account holder see about the additional line? I know they can see call logs, but what about text messages, email, web pages and social media sites? Does the account holder have access to the content of any messages sent or received?

Thanks.

[csquared]

This could be helpful. I would also suggest that you contact the company directly and ask.

[mathnerd]

Thanks. You know I dug around for quite a bit before I posted that and still couldn't come up with any useful links. My google-fu sucks. Really it does. *sigh*