MWI over SIP trunk

I have configured Asterisks as a voicemail server for my softswitch which is working correctly. However, once a voice message is left asterisk is never sending a sip notify back to the metaswitch for MWI.

What do I need to do to get that to work?

I think what I need to be able to do is to set the refer option in the SIP notify when MWI is sent. Is there a way to do that from the SIP.conf file?