I'm confused, there seems to be open base station implementations such as OpenBTS, why is there no SDR based cellphone already I wonder?, as that'd be really neat in my opinion.
It's kind of like asking where there aren't any gpl licensed m.2 ssd flash controllers, the people who design them are getting paid nicely to design and sell a commercial product. It would be cool if there were such a thing, but there is only so much "free" expertise to go around...