Maybe I'm over/under-thinking the requirement, but we use the Notification object to send out messages and escalate for failed processes. The first level, for example, sends an email message and has its Runtime tab use the MRT to execute another Notification object that sends the second level message if it is not acknowledge within the allotted time. The second level Notification also has its Runtime tab with an MRT and time to execute a third level Notification object and on and on as necessary.
Each notification has an ever increasing list of people receiving the email notification. The email list that each notification sends to includes an address that is associated with a pager. I'm not certain what product is genuinely used under the covers to send the page as it is, from my view, just a "special" Outlook user named EPAGE.
While it's true that we do not send a voice message as Willy desired, what we are currently doing is, for us, more than sufficient for mission critical and high priority tasks. About the only issue is that somebody along the notification chain has to acknowledge the currently active Notification object to stop the subsequent escalations.
We also have BMC's Patrol and HP's AlarmPoint as our basic generic default notification software. However we use UC4 for the application specific notification as described above for those that require multiple escalations and acknowledgements.