The biggest problem with "short" DTMF tones - along with short gaps between tones - was determining when two identical digits were in fact two separate ones and not a single tone burst split by some line interruption etc.
I once worked on a voice mail system with DTMF in-band signalling to the PBX for Message Waiting indication, and there were intermittent complaints from some users that they had messages but were not always notified.
After many, many months (actually a couple of years) it was finally determined that the gap between the DTMF tones coming out of the Voice Mail wasn't long enough for the PBX to reliably detect two identical digits following one another.
The gap was fine for different digits, but when the combination of Message Waiting prefix and extension number had a pair of digits somewhere, and the PBX CPU just happened to be busy at that moment (we thought), the Message Waiting wasn't received correctly.
So to sum up, fast DTMF can cause issues ...
Regards
David Clayton, e-mail: snipped-for-privacy@XYZ.myrealbox.com Melbourne, Victoria, Australia. (Remove the "XYZ." to reply)
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