Closing calls via API

Hi,

I'm having some issues closing calls via the API. Below is my Function. The call number is just hardcoded at the moment. But it returns the error "Failed when closing the Call. Detail: No record was found." when quite clearly that Call Number exists. I've tried a few different ones and still the same result.

Is there a step missing?

Public Function CallClosure() As CallResponse

        Dim CallResponse As New CallResponse

        Dim CallCloseRequest As New CallCloseRequest

        Dim _MyAPIService As New ServiceManagerClient()

        CallCloseRequest.sID = GlobalVariables.sessionID

        CallCloseRequest.lEntityRef = "6147"

        CallResponse = _MyAPIService.CallClose(CallCloseRequest)

        Return CallResponse

    End Function

Cheers