Silence calls from everyone but a defined set of numbers? If I'm on-call, I only want to be woken by important calls or text messages.  How can I set up an iPhone to ring during certain hours only for calls or texts from a certain set of numbers?

If it can't be done using vanilla iPhone apps, is there a 3rd party app that could do it?
You could make a silent ringtone your default, and then assign other custom ringtones to each of that certain set of numbers.

See Apple's Purchasing and creating ringtones FAQ for a start.