BCM2835 has two UART interfaces available - one "stable-frequency reliable UART" and one "unstable-frequency not-so-reliable UART". Currently, ZeroPhone re-maps the "stable frequency UART" to the GSM modem, because GSM has a higher priority in the project. Bluetooth, in turn, is re-mapped to the "unstable UART" - and that's where it stops working, I didn't really look further, though here's something that looks like a solution.
Can you help with getting Bluetooth to work in the basic configuration, as it is? Instructions:
- Get a Pi Zero W or a Raspberry Pi 3, upload Raspbian on SD card, boot it and log in
- Add one line in /boot/config.txt: dtoverlay=pi3-miniuart-bt
- Reboot, check that "hcitool dev" doesn't list any Bluetooth adapters anymore
- See how Bluetooth can be brought back to life, and somehow assess how reliable it is.
- Write instructions, which are to be included on ZeroPhone Wiki.
The stakes are, undoubtedly, high, and rewards consist of some Latvian chocolate mailed to people that help the most =) Performance and compatibility tests with various Bluetooth devices (ideally, audio output devices and streaming music to them) and any other insights will be highly appreciated.
UPDATE: "The thread you linked works for a Rpi3, but not the Rpi0w." So, we need to dig deeper - there's some action going on in the linked thread.