Can I use Android Zello with the screen turned off?

First of all, set desired hardware button as a PTT button (Options>Push-to-talk button)
Next, activate desired contact in Zello app. Now when screen is turned off, you can still use hardware ptt button
to send a message to selected contact.
Alternatively, you can use a wired headset button for the same purpose. There's no way to switch between contacts while screen is off so you all messages will be sent to the same contact.

Questions? Submit a new ticket for support-related inquiries.