When you say 'visual voicemail' do you mean a flashing message waiting indication on a button? Visual Voicemail is actually a different feature on the larger display phones which allows you to see and playback messages without dialling into your voicemail and selecting the relevant options.
Are we talking about embedded VM or Nupoint on MiCollab?
1st option and easiest, link the MiVB or MiCollab to an email server and set up email notification on the mailbox.
2nd option. Change the reroute 1st alternative on your original hunt group so that on no answer it will go to someone else's mailbox which is already linked to a phone. Set up a hunt group name tag *mailbox number e.g. *3001, and reroute always to voicemail. When you dial this new name tag hunt group it will go straight to the mailbox. Then point your existing hunt group to this new number on no answer.
3rd and most difficult option, same method as option 2 but this time diverting your original hunt group to a to a multicall key, which is always forwarded to voicemail with a linked mailbox and message waiting indicator button. Set up multicall key 8021 (as 7021 is already in use as your hunt group number). This button can go on anyone's phone (or even a phone that is not logged in / used) on a key that doesn't even physically exist (e.g. Key 70 on ext 3001). It won't ever ring but you just need to set 1 key up for the directory number. Now reroute the multicall key always to the voicemail hunt group. Create mailbox 8021 and put 8021 as the extension number. Now set up your message waiting indicator buttons on all of your phones against DN 8021. Now forward your existing hunt group to his new number on no answer. When a message is left on 8021 is will light the linked message waiting indicator on all of the phones.