When you're using signing or encryption on your SOAP requests, WCF exepects the response to be signed/encrypted too. When the response is not signed/encrypted the message encoder throws a MessageSecurityException. This is perfectly fine behavior, but in interop scenario's can really bug you, because some WS-* implementations don't sign/encrypt Fault messages. Now, because the message encoder throws the exception, you can't get to the underlying SOAP fault. This means that you have no clue why you received a fault in the first place.
To fix this, Microsoft has provided a hotfix. With this hotfix in place you can specify enableUnsecuredResponse="true" in the binding configuration to allow unsecured responses. Unfortunately this means that also valid responses don't have to be signed/encrypted, defeating the purpose of signing and encryption altogether!
As an alternative, you can implement your own message encoder that wraps the encoder that is actually used. In the wrapper you can either store the received XML for use higher up in the call stack, or retrieve the fault and throw a FaultException<>. Without jumping through hoops the latter option does require your wrapper to know about the fault types it needs to handle. With the former option you can handle the exception higher up in the call stack by catching the MessageSecurityException and throwing a new exception with the XML of the message as a property.