When using a WCF proxy client(derived using SlSvcUtil.exe) in a PCL, and referenced in Xamarin.IOS the following server response raises a "Not Implemented Exception":

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
   <SOAP-ENV:Body>
      <SOAP-ENV:Fault>
         <faultcode>s:Client</faultcode>
         <faultstring>The creator of this fault did not specify a Reason.</faultstring>
         <faultactor>Random</faultactor>
         <detail>
            <ns:ECFPFault xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ns="http://RamiCorpSolutions/BusinessObjects" xmlns:ns0="http://schemas.xmlsoap.org/soap/envelope/">
               <ns:ErrorCode>UNKNOWN</ns:ErrorCode>
               <ns:ErrorMsg>Unknown LoginName</ns:ErrorMsg>
            </ns:ECFPFault>
         </detail>
      </SOAP-ENV:Fault>
   </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

However, when the server responds with the following Messagae, the expected Faultexception<TDetail> is raised.

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
   <s:Body>
      <s:Fault>
         <faultcode>s:Client</faultcode>
         <faultstring xml:lang="en-US">The creator of this fault did not specify a Reason.</faultstring>
         <detail>
            <ECFPFault xmlns="http://RamiCorpSolutions/BusinessObjects" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
               <ErrorCode>UNKNOWN</ErrorCode>
               <ErrorMsg>Unknown LoginName</ErrorMsg>
            </ECFPFault>
         </detail>
      </s:Fault>
   </s:Body>
</s:Envelope>

The only difference in the responses(other than general namespacing semantics) is the inclusion of the "FaultActor" element. 

I have referenced the PCL in a Console application and BOTH responses were handled correctly which leads me to conclusively assume the issue is in Mono's implementation of the WCF stack.

Below is the Stack Trace:
StackTrace " at (wrapper managed-to-native) System.Object:__icall_wrapper_mono_delegate_end_invoke (object,intptr)\n at (wrapper delegate-end-invoke) :end_invoke_object__this___object[]&_IAsyncResult (object[]&,System.IAsyncResult)\n at System.ServiceModel.MonoInternal.ClientRuntimeChannel.EndProcess (System.Reflection.MethodBase method, System.String operationName, System.Object[] parameters, IAsyncResult result) [0x00025] in /Users/builder/data/lanes/3412/3cf8aaed/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/System.ServiceModel/System.ServiceModel/ClientRuntimeChannel.cs:460 \n at System.ServiceModel.ClientBase1+ChannelBase1[TChannel,T].EndInvoke (System.String methodName, System.Object[] args, IAsyncResult result) [0x0003c] in /Users/builder/data/lanes/3412/3cf8aaed/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/System.ServiceModel/System.ServiceModel/ClientBase.cs:404 \n at ECFPLib.ECFPService.ECFPServiceClient+ECFPServiceClientChannel.EndLogin (IAsyncResult result) [0x00010] in /Users/ramiayyad/Documents/RDC Connections/Bonobo.Git.Server/ECFP.git/ECFPLib/Service References/ECFPService/Reference.cs:2778 \n at ECFPLib.ECFPService.ECFPServiceClient.ECFPLib.ECFPService.IECFPService.EndLogin (IAsyncResult result) [0x00008] in /Users/ramiayyad/Documents/RDC Connections/Bonobo.Git.Server/ECFP.git/ECFPLib/Service References/ECFPService/Reference.cs:1939 \n at ECFPLib.ECFPService.ECFPServiceClient.OnEndLogin (IAsyncResult result) [0x00003] in /Users/ramiayyad/Documents/RDC Connections/Bonobo.Git.Server/ECFP.git/ECFPLib/Service References/ECFPService/Reference.cs:1949 \n at System.ServiceModel.ClientBase`1+c__AnonStorey0[TChannel].<>m__0 (IAsyncResult ar) [0x00006] in /Users/builder/data/lanes/3412/3cf8aaed/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/System.ServiceModel/System.ServiceModel/ClientBase.cs:242 " string


The FaultContract was defined as follows:

    [DataContract(Namespace = "http://RamiCorpSolutions/BusinessObjects")]
    public class ECFPFault
    {

        [DataMember]
        public ErrorCode ErrorCode { get; set; }

        [DataMember]
        public string ErrorMsg { get; set; }


    }

With a sample interface defined as:
        [OperationContract]
        [FaultContract(typeof(ECFPFault), Namespace = "http://RamiCorpSolutions/BusinessObjects")]
        String Login(string Username, string Password);

Final note,
If you're wondering how the service was implemented, we took the WSDL generated by WCF and implemented in another platform(WSDL first approach).