sinch video sample not calling from android to ios device

Question

I am trying to use sinch sdk in my application, I have created a new app and downloaded the sample and change the configuration of the sample project, all of them works fine except this sample [sinch-rtc-sample-video]

1. between two android devices ==> works fine.
2. try to ring from android to iOS ==> does not work.
3. try to ring from iOS to android ==> works fine.

can you please advice in this problem ?

EDIT

D/Call: onSessionProgressing: 90593d53-8742-49e4-bfca-59d631dfa533: SessionDetails [startTime=1483887776, endTime=0, progressTime=1483887777, establishTime=0, terminationCause=NONE, error=null]
D/CallScreenActivity: Call progressing
D/DefaultHttpService: Http request to https://sandbox.sinch.com/V1/Session
D/Call: onSessionTerminated: 90593d53-8742-49e4-bfca-59d631dfa533: SessionDetails [startTime=1483887776, endTime=1483887823, progressTime=1483887777, establishTime=0, terminationCause=NO_ANSWER, error=null]
D/CallScreenActivity: Call ended. Reason: NO_ANSWER