If it's an incoming call you have the option of picking preset SMS messages or create them by going to Settings>Phone then tap on Respond with text and create your own responses
You can reply to a call with a preset text. But you have to tap the option. If you're too busy to talk, you tap to send the text. But as far as I know, you can't just preset to answer with text when you're not there or sleeping. You have to initiate it when the call comes in. I don't have a clue about any jailbreak option.