There are several models of voicemails that plug in as cards in to the Coral systems. They are:
uCMC - made by Tadiran, the preferred model, small & large type card for all Coral models
WiCMC - software made by ActiveVoice, is just Repartee for Windows on a Coral card, small & large type card for all Coral models
SFC (Simplicity For Coral) - made by ABSTalkx (company part owned by Tadiran owner), small & large type card for all Coral models
iVMF - very basic voicemail, only for IPx500/FlexiCom 200/Coral SL main cabinet
The uCMC is probably the best. WiCMC is quite expensive compared to the uCMC. Both are Windows based so can be added to a customer's domain and backup strategy. Admin is easy through Windows utils.
SFC is DOS based. Quite bif in the US. I'm not a fan. Because it's DOS based admin isn't the nicest.
iVMF is very limited - only 64 mailboxes. It's only really for small systems.
For you scenario I'd got with a uCMC. Anywhere from 4-24 ports. You can make this a centralized voicemail for the site with the Coral SL. I assume you're using QSIG for the link, just divert to a network number (the pilot of the VM)