In a Microsoft Skype for Business Server 2015 environment, you try to transfer audio to a UCWA client such as Skype for Business for iOS during a meeting. However, the audio transfer fails. It's expected that the conference including the audio can be switched to the UCWA client in the correct way.
You can rejoin the meeting from the UCWA client.
To fix this issue, install the January 2019 cumulative update 6.0.9319.537 for Skype for Business Server 2015, Front End Server and Edge Server.