Sometimes you can't take an important call and want to at least send a quick message to acknowledge the call.
On your iPhone, there are three default text messages you can send as responses to calls on your iPhone, but these may not be well suited for you.
You can customize these messages. Click “Settings” on the Home screen on your iPhone. On the “Settings” screen, Click “Phone”. Click “Respond with Text” on the “Phone” settings screen. On the “Respond with Text” screen, tap on the response you want to replace. Type your custom response.
Now when a call comes in while you're tied up, click "Message" above "Slide to answer" when your phone is ringing. You will be prompted to select your choice custom message.