Certainly QSIG can provide MWI as a supplementary service but it is really up to the PBX on whether or not it is implemented. You should be ok on the CCM side (as long as you're not running something really old).
On the Nortel side, I recently helped another site with a Nortel SL-100 where we eventually found Nortel documentation that very explicitly stated it would not do MWI over a PRI trunk, QSIG or otherwise. So if you've got an SL-100 you might need to look for other options. A Nortel CS1000, for example, might have a different story. So again, your best bet is to find the closest match(es) on the PBX Interop page that I linked to. They've been pretty good about including voicemail features (e.g. MWI) in the more recent interop testing. The CS1000 is one they've done a lot of work with.
Two other elements that I suggest watch for when doing such centralized VM deployments are
- Redirecting Number: When forwarding calls the far PBX (Nortel in this case) must pass the redirecting number to the near PBX (CCM) in order to send calls to the correct greeting. For what it's worth, the SL-100 I worked with recently had no problem with this.
- Path Replacement: The default transfer on a PRI trunk will be Transfer by Join. By itself, that'll create a lot of hairpinned calls if Unity is doing much AA transfers. Path Replacement is a supplementary service that'll help free up your b-channels after a transfer is released.
