All questions are in bold for ease of use.
The major carriers in the United States participate in NSA surveillance (except for T-Mobile apparently, because it’s based outside of the US. Except they bought Sprint, which participates.) and that, along with other major privacy issues, means that the market for private carriers is incredibly slim. When I found out that some carriers, such as Mint Mobile, piggyback off of Verizon, I wondered: What’s stopping a carrier from simply E2EE everything from Verizon, and then using Verizon to transfer the data? Obviously, the encrypted data could still be collected and sold, but it wouldn’t matter if the encryption was setup properly, right? I’m looking to better understand how this works, and, if a solution exists, potentially be the first to make it happen. The reason I’m not suggesting creating a carrier without piggybacking is due to the sheer cost and lack of support it would have, which would lead to poor adoption. Also, if carriers simply don’t support E2EE, couldn’t carrier locked phones install the software (since most install software anyways) required to make E2EE work?
As another poster said, the underlying tech is not private: https://jmp.chat/privacy
For backwards compatibility, what your proposing is unlikely unless driven through regulation (personal opinion).
Use something over the top (like Signal was suggested), use a non-KYC provider (like Jmp), or use a burner phone.
A non-KYC provider I wouldn’t trust to be private personally, especially as a secondary SIM. Maybe slightly above average (the company can’t sell the number attached to my name), but I’m sure enough information leaks that a state-level actor could correlate the device to me. The IMEI the tower gets is probably enough to run to Google to figure out who bought the phone.
Even burners may trace back to you through GPS or triangulation depending on how private you really want to be.