Need to email the INBOUND DID of a TRUNK on Voicemail

We need the INBOUND DID of the TRUNK available when a calls comes in on when a voicemail is emailed to a user. THis will be helpful especially because we have lots of inbound DIDs going to a dropbox and it would be nice to know which call came in from where without having to add PREPENDED data to the callerid for each one.

did means ${from_did}
i found
for me it will be
exten => *999,1,Set(CALLERID(name)=${FROM_DID})
but i don`t know where to put it
pls help