To perform this how-to you must be familiar with the command line, as I mentioned in other post ( @klh excuse the off-topic ) I created a service to record all calls.

The first step is run :

arecord file.wav

make a call and then stop recording with Ctrl+C

play recorded call, if voices are not clearly heard unfortunately this how-to is already over

otherwise you can proceed ...

• You have to create callrecorder service, to create it mount root in read/write mode :

sudo mount -o remount,rw /

sudo nano /etc/init.d/callrecorder :

#! /bin/sh

### BEGIN INIT INFO
# Provides:          callrecorder
# Required-Start:    $local_fs
# Required-Stop:     &local_fs
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Call Recorder
# Description:       Call Recorder Daemon
### END INIT INFO

# Written by Br1 <br1@spacelinux.it>

case "$1" in
  start)
    echo "Call recorder starting ..."
    bash -c 'cd /home/phablet/.callrec/ && ./prepare-sh &'
   ;;
  stop)
    pkill manager.sh ; pkill ini.sh ; pkill ter.sh ; pkill ris.sh ;
    rm /home/phablet/.callrec/CallRecorder/.*swp 2> /dev/null ;
    rm /home/phablet/aRam/Rec_Call/.*swp 2> /dev/null ;

    echo "Call recorder stopped !"
    ;;

  *)
    echo "Usage: sudo service callrecorder {start|stop}"
    exit 1
    ;;
esac

exit 0

and set permission with : sudo chmod 755 /etc/init.d/callrecorder

enable autostart with : sudo update-rc.d callrecorder defaults
( if you want disable it : sudo update-rc.d -f callrecorder remove)

• you have to create 3 directories :
  /home/phablet/aRam
  /home/phablet/.callrec
  /home/phablet/MyRec

• and you also have to create 5 scripts :

1 - nano /home phablet/.callrec/prepare-sh :

#!/bin/sh

# ----- start/stop service section -----
#
# check service status
if ps aux | grep -q "[m]anager.sh" ; then echo "    ! Service already running !" ; exit

fi

# check if ramdisk is mounted and exec scripts
if grep -qs '/phablet/aRam ' /proc/mounts ; then cd /home/phablet/aRam/Rec_Call ; ./manager.sh & ./ini.sh & ./ter.sh & ./ris.sh & exit 

fi

# ----- boot section ----- 
#
/bin/sleep 10 ;

# clear cache and buffer
sh -c "free && sync && echo 3 > /proc/sys/vm/drop_caches && free" ;

# create ramdisk
mount -t tmpfs -o size=200M tmpfs /home/phablet/aRam ;

# create directory
mkdir -p /home/phablet/aRam/Rec_Call ;

# copy scripts to directory just created
cp -rp /home/phablet/.callrec/*.sh /home/phablet/aRam/Rec_Call ;

# and exec them
cd /home/phablet/aRam/Rec_Call ; ./manager.sh & ./ini.sh & ./ter.sh & ./ris.sh &

exit

2 - nano /home/phablet/.callrec/manager.sh :

#!/bin/sh

while true

do

FL1=/home/phablet/aRam/Rec_Call/fl1 ;
FL2=/home/phablet/aRam/Rec_Call/fl2 ;
FL3=/home/phablet/aRam/Rec_Call/fl3 ;
RecDir=/home/phablet/MyRec ;
HomDir=/home/phablet/aRam/Rec_Call ;

# if fl1 exist ini.sh intercepted outgoing or incoming call
if [ -f "$FL1" ]; then
	# deletes temporary file and starts recording as phablet user (with .new extension)
	rm -f $FL1 ; su phablet -s /usr/bin/arecord /home/phablet/MyRec/"$(date +%d_%m_%y__%H_%M_%S)".wav.new &
fi

# if fl2 exist call has ended 
if [ -f "$FL2" ]; then
	# deletes the temporary file and stop recording
	rm -f $FL2 ; pkill -f arecord ; sleep 0.2 ;
	# if fl3 exist call was answered
	if [ -f "$FL3" ]; then
		# delete .new extension and exec three scripts
		rm -f $FL3 ; mv $RecDir/*.new $RecDir/"$(date +%d_%m_%y__%H_%M_%S)".wav ;  $HomDir/ini.sh & $HomDir/ter.sh & $HomDir/ris.sh &
		else
		# call was not answered
		# delete recording and exec two scripts
		rm $RecDir/*.new ; $HomDir/ini.sh & $HomDir/ter.sh &
	fi

fi

sleep 1 ;

done ;

3 - nano /home/phablet/.callrec/ini.sh :

#!/bin/sh

cd /home/phablet/aRam/Rec_Call ;

inicmline="dbus-monitor --system interface=org.ofono.VoiceCallManager member=CallAdded" ;

$inicmline | while read line
do
    echo $line | grep "incoming\|dialing"
        if [ "$?" -eq "0" ]; then
        echo "";/bin/touch fl1;echo "";
        break
    fi
done

4 - nano /home/phablet/.callrec/ter.sh :

#!/bin/sh

cd /home/phablet/aRam/Rec_Call ;

tercmline="dbus-monitor --system interface=org.ofono.VoiceCall member=PropertyChanged" ;

$tercmline | while read line
do
    echo $line | grep "disconnected"
        if [ "$?" -eq "0" ]; then
        echo "";/bin/touch fl2;echo "";
        break
    fi
done

5 - nano /home/phablet/.callrec/ris.sh :

#!/bin/sh

cd /home/phablet/aRam/Rec_Call ;

riscmline="dbus-monitor --system interface=org.ofono.VoiceCall member=PropertyChanged" ;

$riscmline | while read line
do
    echo $line | grep "StartTime"
        if [ "$?" -eq "0" ]; then
        echo "";/bin/touch fl3;echo "";
        break
    fi
done

make all scripts executable with chmod u+x

• you can start the service with : sudo service callrecorder start
  or reboot device

-------- overview --- how it works --------

• callrecorder service run prepare-sh script and allows you to stop the execution by terminating 4 running processes
• prepare-sh creates ramdisk in order to speed up read/write operations, copies 4 scripts on ramdisk and executes them, some more details in scripts comments (any text after a hash sign '#')

I was planning to make a single script always running and it didn't work, so I created a four scripts system :

• manager.sh handles three child/processes scripts, each of them intercepts a string (dbus-monitor) creates temporary file and exit

• ini.sh intercepts outgoing and incoming calls

• ris.sh intercepts the start of phone communication (call was answered)

• ter.sh intercepts the end of phone communication

the last two would suffice, however, starting the recording at the beginning of phone communication, sometimes 1/2 seconds are lost ... so I preferred to start the recording on outgoing/incoming calls (if there is no answer recording is deleted)

Calls are recorded in uncompressed format (wav) but if you periodically delete the recordings there will be no problems ... I prefer to convert them (to ogg format) daily with crontab managed script, if you want I attach it, but it will be necessary install ffmpeg (via apt), I will complete it to delete recordings older than 40 days.

I hope I did not forget anything ... English is not my natural language, if something is not clear, you ask and I will try to explain better.

I have not detected abnormal battery drain, maybe these scripts can be improved, anyway callrecorder service works fine on my Xiaomi Mi A2.

I want to say Thanks to all volunteers working on UBports project, having linux on a smartphone is very rewarding