Sound issue when app is launched by Callkit

In iOS, I have implemented the incoming call function using Callkit.
However, when the app is started in response to an incoming call when the app is not running, the operation sounds that should be played when the buttons are tapped and so on do not play though the voice of the call plays normally. This issue persists until app is closed.
Is there anyone with a similar issue? The Unity version is 2018.3.14.