DTMF: False positives

Hi,

I have been a little annoyed for ages about DTMF being detected where none  
has been present.
Now I had a rather interesting occurrence.
I had a false positive while waiting for someone on the other end to fetch  
something. That is there was silence at the other end apart from some  
slight background noise when DTMF was detected.
So that looks to me as if there is no minimum volume level for DTMF  
detection.
Can someone confirm that theory? Or even fix it?

   Greetings,
     WIMPy