Use a VPN (Mullvad is generally the best outside of setting up your own). If you have issues with VPN blocking of services that you need/desire, Private Internet Access with a Dedicated IP is probably the best option.
Set the VPN kill switch option on your phone (remember that you need the VPN in every profile).
The VPN usage will prevent your telecom from knowing what sites you visit and will prevent those sites from knowing what telecom you are using.
The telecom will still have records of every phone call and SMS along with fairly accurate location data thanks to the phone connecting to cell towers.
Establish a Signal account with the SIM number and then refuse to engage with anyone over phone or SMS.
SimpleX is more secure and anonymous but adoption and ease of use suffer compared to signal.
Establish a ProtonMail account, do not link a recovery phone or email to it.
Use authenticator app (Aegis is the best) or that proton email for 2fa. Anything sent over SMS should be assumed compromised against a state actor.
Ultimately, privacy and security are in balance with ease of use and cost. What compromises you make are up to you to decide.
Base GOS (no apps) on a Pixel 9 with a good VPN, USB disabled, and a strong password will be essentially impossible for anyone to access without your assistance. Any phone calls or SMS messages too or from the device should be assumed compromised but the device will be secure.
Going beyond that is a much more complex topic. What apps for you install? In what profiles? How do you use your device and those apps? What threats are you concerned about? What ease of use and cost compromises are you willing to make in mitigating those threats? What are your associates willing and able to do to help you mitigate the threat?
For some people/circumstances appropriate threat mitigation might involve buying property in multiple nations using shell companies with concealed ownership and using those properties to host RAM Disk servers that you are piping all of your internet traffic through. Or hiring your own team of experts to code you up custom apps for your needs.
For other circumstances, buying a Pixel direct from Google with your credit card, flashing GOS to it, and putting in an eSim bought in your own name from Verizon is perfectly fine.