In Shoretel 14.2 and Shoretel Connect there used to be a scenario when a voicemail was not answered in so much time, it would escalate to another user (either emailing or dialing the user and saying "there is an unanswered message" or something like that. There was a ton of flexibility (probably more than needed as setting it up was sort of tedious) and you could control all the times, etc. It was handy for afterhours solutions, or if a manager wanted to keep an eye on voicemails, etc. Shoretel used to allow (and I mention them as I believe you purchased IP rights from them recently?) you to set up like 5 different or more scenarios per extesnion, etc. It was overkill (I could also demo this until we shut down our old system if needed).
So, I think that is maybe too much, just having one escalation scenario would be nice, maybe with multiple users (or just one) and maybe with the option to select how long a call goes before it escalates (I think Shoretel could go up to 10 days or something, it was based on number of minutes). The other thing it was bad at though, was that the weekends were counted, so often escalations came Sunday night from a late in the day Friday voicemail, etc. So ruling out the weekends in the escalation would be a bonus.
This isn't any urgent feature, but I thought it may be something useful to your customers. I had used it in maybe only 1/2 of a percent or fewer of all our users, but I could see some scenarios would benefit more from it. Administration and delegated overall Voicemail Box review would also help alleviate the need for our end, and I have a different feature recommendation already for that.