Our other DBA and I alternate who is on call. The DBA "on call" handles incoming issues during the day and watching monitoring. For off-hours issues they're the first one called but since it's not an official on call (ie not paid for it and don't carry pager) we're not required to respond. If whomever is on call doesn't respond then the other person gets called (which rarely happens). It works well since we're both dedicated and respond if we can. For any time spend working a page we get time off to compensate and our boss is very flexible with that.

Since it's rare to get paged (every few weeks if that) what we have works. My only concern is that at some point either both of us won't be available or something will happen off hours that our monitoring utility catches but since we're not officially on call we won't get paged for and result in downtime only because it's not handled before users come in. However, both my boss and his boss are aware of the concern and say it's acceptable, mostly because every department in our hospital has downtime procedures that allow them to operate in the event of any system failure. Which means if anything does happen I won't need to worry as much about defending the fact that it didn't get caught.