In order to record calls, with drupe or with any other call recording app, the app needs to be able to utilize both the device's microphone and its speaker.
Most phones allow drupe's access to these components, but some devices block the access to the speaker and prevent the option to record its output. We'll be keeping track of this problem, we are trying to find ways to make the call recorder compatible with as many devices as we can.
The recording feature is not solely dependant on drupe. Some phone vendors block this feature and drupe tries to overcome this.
One of the ways is to try the various Recording Configurations we've added to the Settings --> Call Recorder --> Recording Configuration. Give it a try!
This is a constant struggle for drupe to support all phones, Android versions and Launchers. That's the reason drupe includes this feature also in the non-PRO version. We release solutions periodically and hope one of them will work for you as well.
drupe team - firstname.lastname@example.org