You've probably used the 'respond with text' function when a call is coming through that you just can't answer at the time. Maybe you're in a meeting or driving. Or maybe you just don't want to have to speak to a particular person. The templates available are pretty standard: "I'll call you later," "I'm on my way," and "I can't talk right now" are the main staples. However, you can spice things up with this neat little trick.
Go to Settings > Phone > Respond with text to customize your replies. If you're on a long drive, add "driving for the next couple of hours, I'll call you back." If you're in an all-day meeting, you can customize for that, too. Better yet, if you're trying to get a pesky suitor out of your life, why not try "on a date, will call you later."