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:
Category
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Description
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Action
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Customer
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The end user’s MSISDN is not mapped to an Operator.
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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.
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