DTMF Recognition with Speech.Recognition namespace

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recognition

Hello,
I'm trying to get DTMF Recognition to work.
I have a wav file that has a the number 1 recored from my phone. It's repeated 6 times and is recorded in 8K 16 bit linear PCM single channel. No recognition occurs Result = null). If I change the Mode to SrgsGrammarMode.Voice and the grammar to DictationGrammar I do get some random recognition. The wav file plays perfectly in media player.
I've looked all over the web and can't find much samples or help. Hopefully someone here has some experience.
thanks
Jim
Code snippet:

string srgsDocumentFile = Path.Combine(Path.GetTempPath(), "srgsDocumentFile.xml");
SrgsDocument document = null;
GrammarBuilder builder = null;
Grammar grammar = null;

Choices firstThree = new Choices(new string[] { "1", "2", "3" });
Choices nextThree = new Choices(new string[] { "4", "5", "6" });
Choices lastThree = new Choices(new string[] { "7", "8", "9" });

Choices keyPadChoices = new Choices(new GrammarBuilder[] { firstThree, nextThree, lastThree, new Choices("0") });

builder = new GrammarBuilder(keyPadChoices);
document = new SrgsDocument(builder);

document.Mode = SrgsGrammarMode.Dtmf;
grammar = new Grammar(document);

RecognitionEngine.SetInputToWaveFile(@"c:\111111.wav");
RecognitionEngine.LoadGrammar(grammar);
RecognitionResult Result = RecognitionEngine.Recognize();
if (null != Result)
{
StringBuilder Output = new StringBuilder();
foreach (RecognizedWordUnit Word in Result.Words)
{
	Output.Append(Word.Text + " ");
}

}