I think I may have overly confused the situation with my previous post, apologies for that I will try to clarify...
I would have to defer to an echo cancellation expert. I know a little about it, but I don't know enough about internal vs external speaker interaction to speak intelligently about it.
The point is not that there is anything wrong with the echo cancellation in the TT (it works perfectly with it's own speaker), but that there is no way for the TT to control your car stereo's output over an audio cable. Without this ability to control the signal output, it will go into mad echo/feedback loop. If you connected the TT directly to your head unit via a car kit, then it may be a different matter.
I do know that there are software stacks for this very purpose available and that there are other products that use car speakers for the phone audio, so it shouldn't be that difficult of a feature to include.
I don't think there is any other product that will let you have phone audio output from a card stereo that are connected via an audio-in socket. The feature is not missing from the TT, it just cannot be implemented properly over an audio cable.
CDMA (GSM is another) is the protocol that the cell phone uses to talk to the cellular network. That has nothing to do with how the cell phone talks to the TomTom (via Bluetooth).
Firstly, IS-95 (CDMA) and GSM are
very different. IS-95 is not a network protocol (like GSM), it specifies only the air interface of the system. Thus the services offered (and how they are implemented) over a CDMA network are decided entirely by the operator.
Furthermore, there are notorious problems with DTMF over IS-95 networks, mainly because CDMA chipsets utilize lossy compression techniques (optimised for voice) which distort many of the DTMF tones to the point where they are unrecognisable by the receiving system (particularly the tones for 0 and * keys).
However, all of the above is not directly related to your problem, which is related to the BT profile on your phone. If all BT connection profile were created equally, there would be no problem. Unfortunately they are not. Manufacturers (and operators) are basically free to choose what features will be supported/available over the BT connection. If they decide that DTMF signaling will not be supported by the BT profile, then you cannot use that feature from the TT.
That is why the TT reports 'Feature not supported'. TT's BT profile does allow it (that's why other people can use it), your phones BT profile does not.
I might have believed that the phone doesn't support the feature except I know other people that have the same phone and the DTMF keypad works fine. Even TT's tech support acknowledges that "it should work" but they are unable to determine why.
The phone as it leaves the factory does support this, the replacement BT profile installed by your handset provider does not. You could try asking them why, and if/when a replacement BT profile will be available.
Of course, you can simply verify all of the above by using a friend's (who is with a different network) phone. I think you will find that there is nothing wrong with the TT.