I'd love to put my own little cell phone together, but in my area the only reliable provider is Verizon, and I haven't been able to find a CDMA module for Pi or Arduino or otherwise.
You might want to try posting this question on specialist electronics subreddits and forums like r/electronics, r/rfelectronics, r/electronic_circuits/, r/hwstartups and r/hardware. So I guess my advice would be to treat it like a numbers' game :). There's bound to be someone out there who can help you.