From version 2.1, the Meeting Server provides the ability to display an on-screen text message to participants in a meeting hosted on the Meeting Server; only one message can be shown at a time. The duration that the message is displayed can be set, or made permanent until a new message is configured.
For users of SIP endpoints and Lync/Skype for Business clients, the on-screen text message is displayed in the video pane. The position of the message in the video pane can be selected from top, middle or bottom. On screen messaging is also sent to other devices that are using ActiveControl in the deployment, for instance CE8.3 endpoints, and individual Meeting Servers not in a cluster but with the in-call message feature enabled. Meeting Servers in a cluster also support on screen messaging through a proprietary mechanism.
Use the messageText, messagePosition and messageDuration parameters for API object /calls.