Fax Detection - Not working

Hi,

I am using UCMA 4.0 with Lync Server 2013.
I have configured a PSTN trunk with Media bypass enabled and make call from UCMA application
I am unable to detect if the other party is a fax machine with the below code

ToneController tonner = new ToneController();
tonner.AttachFlow(call.Flow);
tonner.IncomingFaxDetected += new EventHandler<IncomingFaxDetectedEventArgs>(tonner_IncomingFaxDetected);

Any help?

-Sivakumar N