[cisco-voip] Can't use Telstralink conferencing service - DTMF tone duration?

[Robert Kulagowski]

Site is in Sydney; running 12.4.9T, H.323 PRI.  Users are complaining 
that once they hear the prompt from the conferencing service, the 
service apparently isn't hearing their DTMF PIN numbers, so they can't 
get into their conference.

If the users route the call out through an analog FXS, it works.

I did a test call, and it sounds like no matter how long I press the 
keypad on the phone, the resulting DTMF is just a "blip"; maybe 1/10 
second long.

What controls how long the DTMF tone duration lasts?  Other sites that 
require PINs (banking, for example), appear to work, but I figure 
increasing the DTMF duration is a quick test.