There's actually another alternative to jail breaking your phone and getting a call blocker application. You can simply get one of those Internet phone numbers. There are a few different applications that allow you to get a phone number and it will ring to your phone, as long as you have the application installed.
These applications usually allow you to block calls, or only receive calls at a certain time. Plus, downloading an application is a lot easier than jail breaking your phone, and it won't void the warranty.