It totally should be able to call the 4 digit extensions. When you set up AAM or MM off ASM, you need to make a dial pattern for your voicemail pilot to tell ASM that those calls should route to voicemail, and you must also make dial patterns matching your extension ranges in CM such that when AAM/MM/3rd party voicemail sends an MWI SIP message that this message is routed to CM to tell it to light the lamp for the station.
So, on the surface, I'd think your voicemail would be leveraging the same ASM programming to say "call 1234" as it would to "turn on mwi for 1234".
That being said, in SM's routing, you setup domains, dial patterns, routing policies and locations. Check into "locations" - you can assign a location to an entity as well as to IP address ranges.
That routing setup works something like:
If you dial 911 from "location" 5, route the call to the routing policy for the little audiocodes gateway with a single analog trunk at location 5. If you did the same from location 6, do the same but to the gateway in location 6.
If you dial the voicemail pilot from any location (except CM), then, send the call to the CM routing policy so it goes through CM's voicemail hunt group. If you dial the voicemail pilot from the CM location, then actually route the call to your voicemail. With a setup like you can always look at things from a design perspective as having the voicemail "behind" the PBX.