Use H264 video codec, if exist (IOS ONLY)
Custom string associated with the call session. It can be passed to the cloud to be obtained from the CallAlerting event or Call History using HTTP API. Maximum size is 200 bytes. Use the Call.sendMessage method to pass a string over the limit; in order to pass a large data use media_session_access_url on your backend.
Optional custom parameter (SIP headers) that should be passes with call (INVITE) message. Parameter names must start with "X-" to be processed. Headers size limit is 200 bytes