Probably the most viable phone OS is Legacy OS, which is just an open source version of Android. You can install it, and provided you don't instal gapps, it is pretty secure. It also gives you complete control over your phone. Legacy OS + Fdroid gives you a FOSS solution that protects your privacy about as well as any OS for a tracking device can.
There is also the openmoko stack from a few years ago. If you can get your hands on a Neo FreeRunner, they are an acceptable phone. You'll be on your own for software though, as that project is effectively dead.
No matter what you do, you can't really trust a phone completely. The nature of the cell network means that any cellphone is a defacto tracking device. Your whereabouts are logged, and because you have shared them with a third party you have no expectation of privacy. They don't even require a warrant for law enforcement. Also, private citizens can simply purchase the location data from most providers. So keep that in mind. I carry a phone, but I am ready to stick it in a microwave and run from it at a moment's notice.