It depands on how it is programmed:
- One way is that extension has a call forward (no answer) set to VM (you can change the timer in MMC 502 (No Ans Fwd)
- Another way is that the call rings on a group, that has an overflow to VM - you have to find the right group, and you can change the overflow time in MMC 601

There is probably some other possible way, so first you have to understand the call flow.