In a synchronous environment, when using WS-Addressing, only services need to have WS-Addressing EndpointReferences. Clients need to to have an implementation that supports WS-Addressing when either (1) the service being invoked is acessed via a WS-Addressing EndpointReference, or (2) the service being invoked uses an EndpointRefernce in a wsdl:message on one of its wsdl:operations. 

In an asynchronous request-response environment, where a service is invoked through an EndpointReference, the WS-Addressing reply destination MUST point to a WS-Addressing EndpointReference to target the response. The reply-to address may or may not point to the client that issued the request.