u'll have to do OS (i.e. ROM) level modificatins. coz android app dev framework might not allow the GSM stream access(at least in non-rooted phone). For ROM level modification we need root access.
I'm not so sure about this. Plz someone correct me if I'm wrong..
The above might also be a reason that stock (pre-installed or given by OEM) calling app have good quality call recording feature (xiaomi, oneplus are good example) than the apps available in market.
So your research can be on whether the latest Android app development api allow the access of GSM stream.
Hope this may helpful in your research.
Additional link-
https://forum.xda-developers.com/showthread.php?t=2607145
Thanks ... Will go through these docs
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