Use the Droid Bionic's Text Message Reply
When you dismiss an incoming call on your Droid Bionic, you may see a pop-up menu similar to the one shown. That’s the Text Message Reply feature in action: Choose an option to send the caller a quick text message, explaining why you didn’t answer.
If the Text Message Reply feature isn’t activated on your Droid Bionic, you can follow these steps to turn it on:
At the Home screen, press the Menu soft button.
Choose Call Settings and then choose Text Message Reply.
Ensure that a green check mark appears by the option Text Message Reply.
You have to scroll down a bit to find the Text Message Reply option.
You can choose the Edit Messages option to remove existing text message replies or to create your own.
The first message shown in the figure is a custom message created by the user. Choose "Create new message" to create your own.
After the feature is activated on your phone, you see the Text Message Reply menu whenever you dismiss or ignore an incoming call.