You need to rely on the interdigit timer for the overlapping plan so that it knows which the final intended destination is. Even if digits for the shorter extension are sent enbloc from the phone, CM will still wait for the T302 timer to pop before routing the call.
You can tweak the T302 timer in CM to lower the interdigit timer to something more comfortable, if that works for your users. Other options are arranging your CSSs so that there isn't overlap, or using a prefix.
You should never, ever have overlapping extensions.
No matter whatever configuration and subterfuges you will adopt to alleviate the problem, the result will be poor, unfriendly to the user, and producing endless issues, as a consequence of the flawed approach taken.
If you customer has objections to that, ask him, do they do that in the public phone system, that has now more than 100 years of history ?
One option around this, which would require the end users to think a touch (good luck) but you could put two line appearances on the phone. One of course is their personal extension lets say is 4000 and it would have access to all the 4 digit extensions in the system through the proper search space. Then on the other line appearance you would label it as 3-digit or something to that affect. Then this line appearance would be in a search space that only recognizes the partitions that contain the 3 digit extensions. So you would have to create new partitions one for the 3 digit extensions and one for teh 4 digit extensions, then a couple of search spaces that contain only the partitions you want the "line appearances" to dial. Then when the users want to dial the 3 digit extensions they would just push the second line appearance button first then dial.
Maybe it doesnt fit your actual scenario but jsut another thought.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...