In iOS, I can't receive incoming call.. (with mp4)

[Problem/Question]

in iOS, I can’t receive incoming call…

I just download your AppStore app and try to make call with iphone
(I also tried with Sample App)

Here’s a situation

Web Widget (dudu)
iOS App (dndn)

call from iOS App to Web Widget working well
but call form Web Widget to iOS App working strangely

Please check the attached video
It works only when app state changed from background to foreground

We really want to use your company’s sdk…
Is there anything I need to do??