You shouldn't have bought the application before testing the free version, because the developers state that you should try the free version to see if recording is supported. What you experience could be a bug or compatibility.
This is what the developers say:
"IMPORTANT: Please try the free version first to make sure that your device supports recording calls."
If you believe it's a bug, reinstall the application and then test it again. If you experience the same issue, it means it's not compatible and the developers should be contacted instead. Use the following email to reach them: