For example, an address could contain ampersands, so it should be wrapped as follows:
<pre> <primarycontact> <firstname>@firstname@</firstname> <lastname>@lastname@</prelastname> <address> <![CDATA[@address@]]></address> </primarycontact> </pre>
If the response to your Web Service call contains values that you wish to store within a Custom Field on the object against which the Workflow was triggered, you can connect a Update Custom Field Value Workflow Task to be the task following the Web Service - SOAP Message and use the variable syntax to pull the value from the XML Custom Field you selected to store the response to the Web Service call.