Hello Amy Khan,
C4C API's you are using are synchronous services which will generate synchronous calls(request-response type). All rejected calls will be shown in webservice message monitoring and few of them will trigger a transaction id in response with which you can create an incident with SAP.
Unfortunately you cannot monitor successfull calls in webservice message monitoring view because the response will be sent to sender system with status.
Regards,
Manideep Satya