I currently use T-Mobile, and welcome any advice you have regarding the choice of operators. The question you pose is a sticky one, and why I note that privacy and mobile phone usage are antithetical. I understand that the metadata/content of voice/mms/sms communications, location info (tower info), and the content of your data communications will always be the domain of the telco and there's no solution to that problem other than abstaining.

I'm not sure how I feel about that thorny issue, and it is almost enough to make me forgo a mobile phone. However, I believe that I derive more value from the ability to communicate than I lose by making that data available to the telco and anyone they cooperate with.

Therefore my goals for privacy are to accept that landscape but try to limit which arbitrary other entities get additional data from my mobile phone usage. I attempt to limit the following things where feasible: oversharing from applications, tracking by other entities such as Google/Apple/etc, installed applications to those providing the best tradeoffs, and the amount of plaintext communication from the phone.

I wish opportunistic end-to-end voice/sms/mms encryption was mainstream, that alone would mitigate a large amount of privacy concerns.