If you can't talk and want to text instead, you can use the Call Alert feature to accomplish this. Call Alert is normally used for emergencies and emits a loud persistent beep, along with an optional text message of 140 characters maximum. The text messages are also saved in history. To use Call Alert as a texting feature, you can turn off the audio beeps by using these steps:
1. Go to Options>Alerts (or Alert tone / vibrate)>Call alert (or incoming alert). Turn alert setting OFF.
2. Tap on the Call Alert icon on the contact's talk screen to text a message: