I wish there was an answer to this..

I have never, ever heard these tones and no one on the other end has ever said they have heard them-cell/land or otherwise.

I use either my SPA2102 or the VOIPo supplied PAP2T....each one set up as they provision it.
ATT is my DSL provider and I have a Netopia/Motorola DSL modem 2210-02-1006 along with my absolutely great ASUS WL-520GU router-STUN disabled..DTMF=auto...strict hold off time=90. Every DTMF driven access works without a flaw..