I'm Busy SMS: it rejects the call and sends an SMS saying you're busy (Cydia)

I'm Busy SMS will add a new slider to lock your screen (when you call it) as you can see in the image above. When you reject an incoming call and automatically send an SMS to the person calling you. You can enable and disable it from Settings and write your own personalized message.

IOS requires 4 +

You can download it for $ 2.99 in Cydia.

You need to be jailbroken.