Consider the following scenario:
You apply Skype for Business Server 2015 Cumulative Update 5 (CU5) to Microsoft Skype for Business Server 2015 and set the MeetingUxUseCdn to True.
You create a Skype meeting and use Skype for Business Web App to join the meeting anonymously.
You click the Switch audio to my phone button and then input your phone number.
In this scenario, you can only hear the ringtone when picking up the call.
Unified Communications Web API (UCWA) doesn't work correctly for this scenario.
To fix this issue, install the January 2018 cumulative update 6.0.9319.514 for Skype for Business Server 2015, Web Components Server.