Assuming you use voicemail at all, this app could just revolutionise it. ON Voicefeed lets you customise voicemail greetings depending on who the caller is. Like the Hullomail app, you can also view left messages as a text feed so you know who's left a message and whether you're going to bother listening to it at all.
You can record a special greeting for, say, dear old mum when it's her birthday and she just might call while you're unable to pick up your phone. Or you could tailor a personal I-can't-pick-up-your-call to a friend, and a rather different one to a work colleague.
You can also divide contacts into groups, and personalise messages for each group.
If you don't use voicemail, this app just might make you start. It's downloadable free from the App Store.