Forward a perticular extension to another extension

I was wondering if its at all possible to force an extension to forward to another extension (all internal). So for example if the extension in question was 123 I always want calls coming from 123 to 456 to go to 789 (or am I over complicating things?)

So you want calls originating from extension 123, no matter what extension was called, to always terminate on extension 789?

That was the case but Im going down a different route now. The reason for this was a for a speaker phone to dial a particular number. Im now trying to reconfigure the speaker so it dials a different number (the number I was after) but I have managed to ban it - so I guess my next question is how do I remove an IP from a black list intrusion detection?