Vivox session is disconnected suddenly after asking for permission in ios

[UE4] [2020.01.30-06.19.32:963][608]Log: Vivox ChannelSession Audio State Change in 8D967740C81580365165105: Connecting
[UE4] [2020.01.30-06.19.33:001][608]Log: Vivox Join success
[UE4] [2020.01.30-06.19.33:214][613]Log: Vivox User 8D967740C8 has joined channel 8D967740C81580365165105 (self = false)
[UE4] [2020.01.30-06.19.33:215][613]Log: Vivox Participant Updated (audio=1, text=0, speaking=0, id=8D967740C8)
[UE4] [2020.01.30-06.19.33:774][626]Log: Vivox ChannelSession Audio State Change in 8D967740C81580365165105: Connected
[UE4] [2020.01.30-06.19.33:775][626]Log: Vivox User A948944A38 has joined channel 8D967740C81580365165105 (self = true)
[UE4] [2020.01.30-06.19.33:775][626]Log: Vivox Self Participant Updated (audio=1, text=0, speaking=0)
[UE4] [2020.01.30-06.19.33:775][626]Log: Vivox Participant Updated (audio=1, text=0, speaking=0, id=A948944A38)
[UE4] [2020.01.30-06.19.48:858][982]Log: Vivox ChannelSession Audio State Change in 8D967740C81580365165105: Disconnected

Custom log of my application.
As you can see the log, Vivox has joined channel successfully (already requested permission and allowed at this moment) after then channel session has disconnected suddenly without any verbose log.

If I can bind permission checking in ios, I can resolve this issue by logging in and connecting again probably but 4.22 doesn’t support checking permission in ios.

Please help thanks.