NuPoint mailboxes can do message notification with at least 3 levels (maybe more, but I only ever had to set it up with 3). Look at the Message Notification tab for the mailbox in the NuPoint Web Console. You'll have to set aside a couple of ports for notification callouts. At my last gig we had customer that was an apartment management company that used this for maintenance calls. Resident would call in and leave a message, mailbox would call out to the on-call maintenance tech's cell phone. If they didn't respond within 10 minutes, escalation kicked in and called their manager.
