When I was working in the mobile industry, I was told repeatedly that a 100% FOSS phone would be impossible. Not only is the existing tech proprietary and patent-encumbered, even if somehow you could get past that your GSM radio must be certified before going to market. And if your radio is FOSS-based that means an end-user can toy with the baseband, therefore you will never get your cert, period. The stuff at the lowest level must locked down before being allowed on a public mobile network.