How do I prevent incoming calls from interrupting text message composition on iPhone?

If I am typing a text especially if I am sending a picture with a description. While I am in the middle of typing all the information, my cell phone will ring. It will kick me out of the screen I am in and I loose everything I typed thus far. How do I keep a call from coming in and taking over everything and making me loose everything I worked so hard on!

[Re-Titled by Moderator]