I was able to figure this out. If anyone is interested, I following the below tutorial to get voicemail using FreePBX, while using phones registered to CUCM:
http://www.uccollabing.com/2016/07/06/asterisk-pbx-as-voicemail-for-cucm/
As for integrating MWI, I added this shell script here:
I meant extensions_custom.conf not extensions_additional.conf as they would get overwritten.
The Voipinfo page below had a log of good information:
http://www.voip-info.org/wiki/view/Asterisk+Cisco+CallManager+Voicemail+Integration
This is the script that I am using (mostly from the link above) to turn mwi on and off. Just add the script location to externnotify and MWI should work.
#!/bin/bash
CONTEXT=$1
EXTEN=$2
NUMVMS=$3
Turnoff=“Channel: SIP/1002@callman\nCallerid: $EXTEN\nMaxRetrie…