Modern mobile phones do not only have a main operating system (such as iOS and Android), but also have a second, independently operating component, the so-called baseband processor, that manages communication with the hone network and that also runs software. This software can often be surreptitiously updated over the phone network without the user knowing, its source code is not available for inspection and it can access all components of the phone, such as the microphone or the GPS unit.
Even when you have your main operating system secured against attackers, criminals and/or state agencies might therefore still be able to exploit your phone.
Some people on Indiegogo are not crowdfunding the design of a free baseband design that can run free software. They need USD 7500 for the first phase of the project, so this seems eminently doable and a worthwhile project.