I did not know this existed, and I haven’t tried it, and I can’t even find the original source since it pulled up randomly from somewhere on my Reader Suggestions feed, but here’s the text. Sorry for no source:
If you are busy and can’t take an incoming call on your iPhone, or you are already in a call and can’t take the second call, you can press a shortcut to send an SMS with a preset ‘I’ll call you back’ message to the person calling you.
Simply press Volume down + Sleep button while the phone rings (again, this can be done to an new incoming call or when you are already having a phone conversation and a second person is trying to reach you), then it will confirm the phone number, and just have to press send. The number is already in there, but the numeric keyboard comes up in order to change the recipient or make some modification to the number.