I built a paging system using Freeswitch for supporting our customers 24x7. A couple years later our IT staff wanted to normalize on pagerduty across our company (other departments needed pagers). After using it for a couple months we decided to switch back. I had other friends who wanted to use it as well, so I turned my little side-project into a dead-simple pager service. Check it out and let me know what you think: http://arctheweb.com/