The iOS version of Zello does not support the option to automatically set your status to Busy when your device is silenced.

If you do not wish to receive incoming messages in real-time, adjust your status to one of the following:

• Busy: Audio will be muted and all messages will be saved to History.
• Offline: Missed messages from contacts will be queued for playback next time you set your status to Available. Missed messages from channels will not be saved.

For step-by-step instructions on changing your status, please reference this support article.