As per the official documentation here: Choosing a Message Encoder, WCF's default TextMessageEncoder only supports UTF-8, UTF-16:

TextMessageEncodingBindingElement and MtomMessageEncodingBindingElement support only the UTF8 and UTF16 Unicode (big-endian and little-endian) encodings. If other encodings are required, such as UTF7 or ASCII, a custom encoder must be used. For a sample custom encoder, see Custom Message Encoder.

And there is a link to a custom message encoder sample: Custom Message Encoder: Custom Text Encoder (it uses the ISO-8859-1 encoding, but you get the idea)

See also this blog post: Text Encoding and WCF