Blue POT has been running for some months and, it turns out, only a partial success.  It works but there have been some glitches.

I live in the foothills of the Rocky Mountains so our cell phones have to use WiFi when at home since there is no cell coverage.  My partner's iPhone seems to have a difficult time using both WiFi and Bluetooth when Blue POT is far away from the iPhone.  It drops the WiFi calls.  I theorize this is because the bluetooth system is struggling to move all the voice data and requiring a lot of on-air time and this interferes with the WiFi enough that it can't maintain a voice connection.

Hand wired Blue POT's poor EMI design also manifests.  You can hear clicks whenever the Bluetooth radio goes on-air in the handset and those clicks are louder as the Bluetooth module increases output power.

Finally I have seen some cases where the Bluetooth module won't automatically re-pair with the phone so I suspect there is more code to be written.

Introducing Blue POT Explorer

Undeterred by all that I have pressed on to an improved and more capable design I call Blue POT Explorer.  I have PCBs now and am just starting to look at the code.  Blue POT Explorer is designed to be a flexible platform for developing applications that use unmodified POTS telephones.  It adds the following items.

1. Move to Teensy 3.6 for increased processing power/memory and SD-card for storage of audio clips that can be played over the phone (imagine dialing a number to hear an audio clip).
2. Touch LCD for a GUI for operation and configuration.  I'm planning to use littlevgl for a multi-tile type interface (slide tiles left and right for various screens).  A light sensor/proximity sensor should allow intelligent dimming and display content control.
3. Local speaker and microphone connected to the 2nd set of built-in DAC/ADC interfaces on the Teensy 3.6 for speaker-phone and intercom functions.
4. Optional Ethernet interface for applications like SIP VOIP or remote streaming.  I'm hoping that the ESP32 WiPhone group will release their SIP code and I can adapt it for Blue POT Explorer.

Blue POT Explorer will be documented in a new hackaday.io project and the hardware and firmware will be open-source with a complete BOM and parts sourced from Mouser, Adafruit and Amazon.  It's designed using through-hole parts and modules containing the SMT parts for easy assembly.  The only hard part is soldering the BM64 Bluetooth module and that's not too difficult.