Tom, other interested people,

I finally got it working on a raspberry pi-4 with buster lite, mostly thanks to the following articles:

https://sigmdel.ca/michel/ha/rpi/bluetooth_01_en.html
https://sigmdel.ca/michel/ha/rpi/bluetooth_02_en.html
https://sigmdel.ca/michel/ha/rpi/bluetooth_n_buster_01_en.html

The exact reproducible  steps have been:

1. Started on the basis of
   
   
   + my FreeSwitch installation based on instructions under https://freeswitch.org/confluence/display/FREESWITCH/Raspberry+Pi
   
   
2. sudo apt-get update  &&  sudo apt-get install -y bluealsa
   
   
3. sudo vi /lib/systemd/system/bluealsa.service
   --> substituted ExecStart=/usr/bin/bluealsa with
   -->  Environment=LIBASOUND_THREAD_SAFE=0
      ExecStart=/usr/bin/bluealsa -p hfp-hf -p hsp-hs -p hfp-ag -p hsp-ag -p a2dp-source -p a2dp-sink
   
   
4. sudo vi /lib/systemd/system/bluetooth.service
   --> Added --noplugin=sap to the end of the following line
      ExecStart=/usr/lib/bluetooth/bluetoothd
   
   
5. sudo vi /lib/systemd/system/bthelper@.service
   --> Added ExecStartPre=/bin/sleep 2 in a new line after
      Type=simple
   
   
6. Created a new file by sudo vi /etc/asound.conf and added the following as its sole content (if you know the MAC address of your Bluetooth device, substitute it already now; otherwise you can change it also later in step 9):
   pcm.btAudioIO {
           type asym
           playback.pcm {
                   type plug
                   slave.pcm {
                           type bluealsa
                           service "org.bluealsa"
                           device "00:11:09:94:14:3D"
                           profile "a2dp"
                           delay 10000
                   }
           }
           capture.pcm {
                   type plug
                   slave.pcm {
                           type bluealsa
                           service "org.bluealsa"
                           device "00:11:09:94:14:3D"
                           profile "sco"
                           delay 10000
                   }
           }
           hint {
                   show on
                   description "Bluetooth Audio Input/Output Dvice"
           }
   }
   
   
7. sudo adduser freeswitch audio
   sudo adduser freeswitch bluetooth
   sudo reboot
   
   
8. After reboot, run sudo bluetoothctl and then find, pair, connect and trust your Bluetooth device with the following control commands of this program:
   agent on
   scan on
           <-- find the MAC address of your device in the scan outputs
   pair 50:11:2D:06:ED:2F
   scan off
   connect 50:11:2D:06:ED:2F
   trust 50:11:2D:06:ED:2F
   exit
   --> from now on, your device should automatically reconnect to the pi each time after the pi is rebooted; if, however, this is not the case, you will have to redo this step after each reboot.
   
   
9. In this example, the device that I have paired and trusted has a different MAC address as the one in /etc/asound.conf; therefore we must edit the file and set the right MAC address. If you already had the right MAC address, you may go to the next step without reboot; but, if you had to change the file /etc/asound.conf, then you will have to reboot before going to the next step.
   
   
10. Test your device with the following two commands (five seconds recording):
    arecord -D btAudioIO -d 5 test.wav
    aplay -D btAudioIO test.wav
    
    
11. Assuming that the above test is successful, you now
    - edit <FreeSwitch-Config-Home>/autoload_configs/modules.conf.xml and un-comment the loading of mod_portaudio
    - edit <FreeSwitch-Config-Home>/autoload_configs/portaudio.conf.xml and substitute the whole content with the following new content:
    <configuration name="portaudio.conf" description="Soundcard Endpoint">
      <settings>
        <param name="indev" value="btAudioIO"/>
        <param name="outdev" value="btAudioIO"/>
        <param name="ringdev" value="btAudioIO"/>
        <param name="hold-file" value="$${hold_music}"/>
        <param name="dialplan" value="XML"/>
        <param name="cid-name" value="$${outbound_caller_name}"/>
        <param name="cid-num" value="$${outbound_caller_id}"/>
        <param name="sample-rate" value="48000"/>
        <param name="codec-ms" value="20"/>
      </settings>
    </configuration>c
    
    
12. In my case, I didn't need to do anything for dial plans because in my system both the in- and outbound calls are first handled by my python scripts; in particular, the BT handsfree device is never supposed to make calls but is called for making announcements or invited to a running call as the third leg. So far, I was using an IP telephone with auto-answering capability as extension 000 of my FreeSwitch for this purpose. To substitute that phone with the BT handsfree device, I just needed to update my related python script with the following command:
    sudo sed -i 's#user/000@[0-9.]+#portaudio/auto_answer#g' dial.py
    
    

After a reboot, after which also FreeSwitch and my application start, everything was working for me just fine :-)

Cheers,

-- Saied