Flex does not support SOAP1.2

Flex 3 only supports SOAP1.1 and does not support SOAP1.2: document

Flex applications can interact with web services that define their interfaces in a Web Services Description Language 1.1 (WSDL 1.1) document, which is available as a URL. WSDL is a standard format for describing the messages that a web service understands, the format of its responses to those messages, the protocols that the web service supports, and where to send messages. the Flex web service API generally supports Simple Object Access Protocol (SOAP) 1.1, XML Schema 1.0 (versions 1999,200 0, and 2001), and WSDL 1.1 RPC-encoded, RPC-literal, and document-literal (bare and wrapped style parameters ). the two most common types of web services use remote procedure call (RPC) encoded or document-literal SOAP bindings; the termsEncodedAndLiteralIndicate the type of WSDL-to-SOAP mapping that a service uses.

Flex 4 is not supported either. For details, see Adode Forums.

Therefore, only SOAP.1 can be used for communication between Flex and WCF, and SOAP1.1 is used for basicHttpBinding by default.