Using an iPhone, you can reply with a text message to an incoming call by pressing the "Message" button and selecting the text to send. You can compose a personalized message by choosing the "Customize" option; in addition, through the Device Settings application, you can customize the default messages that appear when you press the "Message" button.
Steps
Part 1 of 2: Reply with a Text Message

Step 1. Wait to receive an incoming call on the iPhone
You will only be able to text a phone call when someone calls you.

Step 2. Press the "Message" button
It is visible within the incoming call screen. It features a cartoon icon and is located on the right side of the screen, above the button to answer the call.

Step 3. Select one of the predefined text messages
You will have the possibility to choose from a list of three ready-made messages. Tap what you want to send to the person who is contacting you.
You can customize these messages as you wish by following the steps described in the next section of the article

Step 4. Choose the "Customize" option to write a message on the fly
This will display the Messages app on the screen, allowing you to compose a personalized message to send to the caller.
Part 2 of 2: Editing the Default Messages

Step 1. Launch the Settings app
It is located on one of the pages that make up the Home screen. In some cases it is located inside the "Utilities" folder.

Step 2. Choose the Phone option

Step 3. Select the Reply with message item

Step 4. Tap one of the text messages that appeared

Step 5. Type the text you want to use to answer incoming calls

Step 6. Tap the X icon to delete one of the already existing messages

Step 7. Select one of the new custom messages to answer an incoming call
The messages you entered in the previous step will have replaced the default ones and will appear when you press the "Message" button. At this point select what you want to send to whoever is calling you.