Symptoms
Assume that you have a Unified Communications Managed API (UCMA) application that sends a dial-out request to the Audio Video Multipoint Conferencing Unit (AVMCU) in Microsoft Skype for Business Server 2015. Then the dial-out goes to a UCMA back-to-back (B2B) application, and is finally sent to a Skype for Business user. When you try to add a video to the dial-out conference, the video can't be added by the B2B application.
Cause
The escalation, which is from an audio to video conference, fails because the addEndpointMedia level achieves an incorrect state from MediaUASession.
Resolution
To fix this issue, install the January 2018 cumulative update 6.0.9319.514 for Skype for Business Server 2015, Conferencing Server.
