First, the front-line pager duty is usually hit by an ops eng team first. Each of those teams has an India group and an North America group. If they can't resolve the issue, they page in the developers. Having good Ops Eng guys supporting you is a blessing.
There's also a very strong incentive for teams to write good software and test before deploying when you know damn well that if you're deploying something that doesn't work, you're the guy who will have to deal with it at 3am. Having strong integration and system tests suddenly becomes incredibly important.
Edit: and as dsfyu404ed said, the Ops Eng guys will be smart enough to narrow down where the problem is usually. If you're being paged, it's usually your team's fault.