Everything is much simpler. I made the instructions in Russian. It is available at the link
Instructions
There in detail with screenshots, here I will write briefly
Regular recording of calls from a line without root on Indian firmware!
It is possible to automatically record all calls, as well as individual calls when a button is pressed during a dialogue. There is no voice notification about the start of recording.
Calls are recorded, including from the headset, for activation it is necessary:
1. Use Indian firmware, for example A.15 (India);
2. Install a dialer (contacts) called OnePlus Dailer
3. In the settings, go to "Default Applications" > "Calls" and select "Contacts"
4. Open the OnePlus Dailer application (calls), go to "Settings" > "Call recording" (this item appears only after you set this application as the default dialer, see point 3) > activate automatic recording.
5. We use a convenient dialer with call recording. Records are available directly from it and are linked to each contact.
Calls are recorded in this way, including from the headset, even if you switch from phone talk mode to headphones, recording will continue seamlessly.
There was a similar message in the subject, but without specifics. And there is a way to install additional software, but why is it if there is enough dialer?
Thanks to those who share information.
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Separately, I note that you are responsible for recording calls in accordance with the legislation of your country, somewhere this may be requested.