A prerequisite for the Live Call Modification API is to know the CallSid of the call. The CallSid is generated by teleapi for every incoming or outgoing call. When we create an outgoing call using the Calls REST API, teleapi will generate the CallSid and the response will contain the CallSid. For an Incoming call, teleapi will generate the CallSid, and will include it in the parameters of the GET or POST request to download the RCML from the application server. So the application server, will get the CallSid for this incoming call and can store it for later use.