Libreo

https://blog.torproject.org/blog/mission-impossible-hardening-android-security-and-privacy

The future is here, and ahead of schedule. Come join us, the weather's nice.
This blog post describes the installation and configuration of a prototype of a secure, full-featured, Android telecommunications device with full Tor support, individual application firewalling, true cell network baseband isolation, and optional ZRTP encrypted voice and video support. ZRTP does run over UDP which is not yet possible to send over Tor, but we are able to send SIP account login and call setup over Tor independently.
The SIP client we recommend also supports dialing normal telephone numbers, but that is also UDP+ZRTP, and the normal telephone network side of the connection is obviously not encrypted.
Aside from a handful of binary blobs to manage the device firmware and graphics acceleration, the entire system can be assembled (and recompiled) using only FOSS components. However, as an added bonus, we will describe how to handle the Google Play store as well, to mitigate the two infamous Google Play Backdoors.