As I gather from your question the input for the webservice call should be a reference number (int) and the return should be the phoneNumber (int). This can be done by creating a microflow like below:
The input for the microflow is an integer.
In the retrieve action you retrieve the object based on a database query via xpath like [ReferenceNumber = $ReferenceNumber], make sure only 1 record is returned.
The return value is set in the end action as an integer and in the example is set as $wizardstep/phoneNumber
The use the option (from the context menu in the MF) to publish the microflow as a web service operation and you’ll end up with a webservice like below:
As for the target namespace make sure it’s something unique, for more information on this see https://stackoverflow.com/questions/28008214/targetnamespace-and-namespace-in-wsdl