iPhone doesn't come with very basic phone feature such as conditional forwarding, e.g. when you are busy/unreachable/not answered etc. These features may be considered by Apple as too basic and your carrier should tell you how to do it by keying in codes at the dial pad.
Found this very good reference. I have just tested this OK with my carrier and I just use the dumb way of simultaneously perform ALL conditional forwards such that when my phone rings I can decide to take it or forward it.
I think someone should design an app to do this smartly.