This is it! Worked perfectly, thanks!
For anyone else looking, the line item for the Sangoma phones is P33:
MWI and VM hard button working with a shared VM box + Sangoma Phones
Extensions>Advanced
Mailbox ####@device (your shared VM)
Voicemail Extension *98#### (your shared VM)
Endpoint Manager>Basefile Edit>Sangoma>S### (your Sangoma phone type)
<P33>__voicemail__</P33> # Voice Mail UserID(UserID for voice mail system)
Changed to
<P33>__voicemailExt__</P33> # Voice Mail UserID(UserID for voice mail system)