The service contract:

[OperationContract]
[WebGet(UriTemplate = "{addressBookId}?action=WipeOut")]
System.IO.Stream StartWipeout(string addressBookId);

// both the Get and the Invoke share the same UriTemplate
[OperationContract]
[WebInvoke(UriTemplate = "{addressBookId}?action=WipeOut")]
XElement DoWipeout(string addressBookId);


The implementation:

public Stream StartWipeout(string addressBookId)
{
MemoryStream ms = new MemoryStream();
StreamWriter sw = new StreamWriter(ms, Encoding.UTF8);
sw.Write("<h1>This will clear PDS and AddressBook data for " + HttpUtility.HtmlEncode(addressBookId) + "</h1>");
sw.Write("<form method='POST'><input type='submit' value='Do wipeout' /></form>");
if (null != WebOperationContext.Current)
WebOperationContext.Current.OutgoingResponse.ContentType = "text/html";
sw.Flush();

ms.Position = 0;
return ms;
}
public XElement DoWipeout(string addressBookId)
{
WipeoutAddressBook(addressBookId);
WipeoutPdsPrefs(addressBookId);
return new XElement("WipedOut", addressBookId);
}

The service contract:

[OperationContract]
[WebGet(UriTemplate = "WipeOut/user={userId}")]
System.IO.Stream StartWipeout(string userId);

// both the Get and the Invoke share the same UriTemplate
[OperationContract]
[WebInvoke(UriTemplate = "WipeOut/user={userId}")]
XElement DoWipeout(string addressBookId);


The implementation:

public Stream StartWipeout(string userId)
{
MemoryStream ms = new MemoryStream();
StreamWriter sw = new StreamWriter(ms, Encoding.UTF8);
sw.Write("This will clear all data for " + HttpUtility.HtmlEncode(userId) + "");
sw.Write("");
if (null != WebOperationContext.Current)
WebOperationContext.Current.OutgoingResponse.ContentType = "text/html";
sw.Flush();

ms.Position = 0;
return ms;
}
public XElement DoWipeout(string userId)
{
// actually do the WipeOut
return new XElement("WipedOut", userId);
}

“<Contact> <ContactId>10</ContactId> <Name>ABC S</Name> <PhoneNumber>555-0001</PhoneNumber></Contact>”

10ABC555-0001

… this POST request never hit the “expected” endpoint. Could you give me enlightenment? Is there any special thing with XML format to adhere?

It is not possible to generate a proxy for REST services.  You can generate wsdl for the service though since the contract still uses ServiceContract and OperationContract attributes.  However you cannot use this metadata to generate a proxy to a REST endpoint.  You can still use it to talk to any other type of endpoint though.  You could have your service exposed via REST and wsHttp.  You could also use this metadata as a means to understand the POX returned from a REST service.  REST just returns the pure XML representation of the objects (unless you specify a different format such as JSON).