Call Alert is a very useful feature to get a hold of someone quickly in case of an emergency. The icon is a box with lightning bolt located next to the big round talk button:
You can use Call Alert in 3 ways:
1. Tap on the Call Alert icon to open a text window and type a text message before sending. Once sent, the recipient will hear 3 loud beeps in succession and will see your text message. The beeps are repeated every 1 minute until responded to. To respond, the recipient can either talk, text, or dismiss the Call Alert.
2. Long press on the Call Alert icon to immediately send a default message "Zello me! and persistent audio alerts to the contact.
3. To use Call Alert texting without audio alerts, go to Menu>Options>Audio Alerts section and turn off the Incoming Alert setting.