The same probably goes for other MVNO carriers such as Mint and Ting. The PII and billing data is with the MVNO carriers. I buy my SIM cards anonymously. I never use cellular near my house and only use it for data over a VPN. So it would not affect me if all of their data was breached.
> I buy my SIM cards anonymously. What's the methodology for doing this successfully?
Use virtual card from service such as privacy.com to add funds.
Never make calls or SMS with the SIM card number. Instead use VOIP such as jmp.chat or voip.ms.