Yes the change is immediate, but it almost sounds like it's ARS digits dialled might not have the correct number of digits in it for what it should be matching. Failing that there is a timer in Class of Service for the devices that will control parts of the timer as well.
Not being in the office at the moment I can't look at the specific timer, but if you know it is 3 seconds, then you should be able to spot the timer setting fairly easily.
