4503 is a SMS 365, enterprise service status code indicating there is no operator found for the current MSISDN.
Reasons for a 4503 response
The reasons why the 4503 status code is returned, and the actions which should be taken, are described below:
The end user’s MSISDN is not mapped to an Operator.
The customer should contact the end-user to confirm that they are using a correct MSISDN and that the number is a valid full international format mobile number, for example, +91234657890.