Absolutely.
The short answer is turn up QSIG on those H323 trunks.
The long answer is that if you have a standalone 450 with trunks, it's got another CM so you're trying to put 2 PBXs on one voicemail and there are considerations.
If you've got a Session Manager between CM7 and AAM, I'd say you should just hang the other system off of Session Manager, manage the dial plan in Session Manager, and AAM can deal with that.
If you only have h323 trunks off the G450 system, you need to get it to go through CM7 to AAM. You need to get MWI back over to the G450. You need to get calls from the G450 to the voicemail pilot with the extension that was called so it knows who's greeting to play and ideally the calling number of the person leaving the message. QSIG can do that where straight H.323 can't.
If you're just looking for a basic answering machine it'll be easier than adding bells and whistles like findme/followme, 0 out to my cell or my assistant. Managing outcalling for mailboxes on PBX1 to use PBX1's trunks and PBX2 to use PBX2's and all that jazz certainly adds a layer of complication. There are plenty of features that would take a lot of fine tuning to make all work properly, but regardless, the basics can be done rather easily.
If you want to give it a shot, try this:
add another h323 sig/trunk on both systems that you'll only route voicemail calls over.
Enable qsig by making "tsc supplementary services protocol" B instead of A, and make sure CA and NCA TSC are >0
In "system-parameters features", give a QSIG TSC extension - its just a number in the dial plan of each system for qsig to use
If your AAM is already up and running voicemail for CM7 and you just want to shoehorn your other CM in there, if you follow this config note below and just make it CM remote-->CM core-->AAM instead of the note's intent of CM via H323 to MM, you'll probably get what you're looking for.