A developer uses the call() instance method of dw.

The initServiceClient callback method in the dw.svc.ServiceCallback class is specifically required when invoking a SOAP service because it is used to initialize and configure the SOAP service client before making the service call. This method sets up necessary configurations such as SOAP headers, namespaces, and other SOAP-specific settings that are not required for RESTful services. Other methods like createRequest, mockCall, and parseResponse are used in both SOAP and REST services but initServiceClient is unique to SOAP due to its specific requirements for client initialization.