Open Settings on your phone
Go to About Phone
Tap Build Number seven times
You will see a message saying Developer Mode Enabled

Go back to Settings
Open Developer Options
Turn on USB Debugging
Scroll down and enable Disable Permission Monitoring

Download OnePlus Call Recorder Pro on your computer
Make sure the files 1.apk and 2.apk are in the same folder as the tool

Connect your phone to the computer using a USB cable
If your phone shows a permission message, tap Allow

Open the tool on your computer
Click 1. CHECK ADB DEVICE
Click 2. INSTALL DIALER APKS
Click 3. ENABLE CALL RECORDING
That’s it. The call recording button will now appear in your dialer.