Get a burner phone with a removable battery that doesn’t have a camera (or cover it) and only use public WiFi.
This advice really depends on what you need to do. Most people in this boat would benefit more from using something like GrapheneOS that provides actual security. If you’re talking about cheap burner phones you can buy for a bit of cash, those will be running proprietary OSes and using entirely unencrypted comms like SMS. Besides, their association with people like drug dealers means that I wouldn’t be surprised if some of them come with purpose-built spyware preinstalled. The burners will work for some narrow use cases, or if you can’t get your hands on a Pixel phone, but for most people with state level threat models it’s poor advice.
This advice really depends on what you need to do. Most people in this boat would benefit more from using something like GrapheneOS that provides actual security. If you’re talking about cheap burner phones you can buy for a bit of cash, those will be running proprietary OSes and using entirely unencrypted comms like SMS. Besides, their association with people like drug dealers means that I wouldn’t be surprised if some of them come with purpose-built spyware preinstalled. The burners will work for some narrow use cases, or if you can’t get your hands on a Pixel phone, but for most people with state level threat models it’s poor advice.