It sounds like you are hitting bug CSCuw81520.
I came across this issue in a lab on CUCM version 11. When you start entering meeting number or access code the T302 timer starts after first digit and doesn't refresh on subsequent digits. The end result is that a user has the T302 timer of time to enter the entire string which could be problematic if T302 is set to a low value like 5 seconds.
You don't have much choice here. You can either increase the T302 timer making inter-digit timeout with overlapping patterns, if any, more noticeable, or you could upgrade CUCM to 11.5 where this bug is purportedly fixed.
