Nagios does that too. You can define schedules to your heart's content, and it will send email anywhere. Given that basically all pagers/mobiles have email addresses, you're set.
The only thing that this does that is really new is two-way SMS.
The only thing that this does that is really new is two-way SMS.
I also think PagerDuty's ability to graphically define the on-call schedule and escalation rules is much nicer than mucking around with Nagios's configuration files, but I'm a bit biased :)