My first step for this kind of thing is to get onto the MCD "Maintenance Commands" and run a "locate feature extensions nnnn".
This will tell you whether there is any Call Forward active.
If not, then I would check the DID form to make sure that the full dialled number is being sent to the correct extension.
Assuming the DID form is OK, I would then check Call Rerouting to see what is configured there for the extension.
Finally, I wouldn't necessarily take at face value that the call is being "forwarded" - maybe both phones are in the same ring group? Maybe the other phone has a line presence for the first phone?
I would also want to know if the users were using MiCollab at all. If so, I would also check the MDUG form to understand what was in there for this extension.