OK I will try to take it step by step
1) create the following short code
Short Code: 666
Numebr: "#<Mailbox Name>" - # means leave mail
Feature: VoicemailCollect
2 Create a pahntom user (a user without a physical extn., set forward unconditional to 666, enable forward hunt group calls. A call to the phantom user will now take a msg for the mailbox
3)creat a new group - set to hunt & add the pahntom usere
4) in your main group add the new group into the overflow group list
calls will ring the group, when they overflow they will call the phantom user, which is on divert & will route dircetct to the specified mailbox.
As I said it is SNEAKY programming but it works!
If you are still stuck I can creat a sample Cfg for you to download.