Realistically the only way I can think of to have a purely open source phone, is to not implement it as a mobile phone at all. You can do voice and interface to the pstn with a purely open source asterisk and sip softphone software combination. If you're a super gpl zealot, do it on one of the few thinkpads that can have its bios flashed with a open source version.

Sorry it's not very portable, beyond whatever subnotebook sized x86-64 system you can run it on. Where your phone calls meet ss7 and the pstn is still going to be totally closed source and opaque to you.