You can't do it at any time. The only time the flow is listening for caller input is during a Collect Input action (also during menus, but in-queue flow don't have menus). You can offer to transfer them to voicemail periodically, for example put a collect input action followed by playing hold music for a minute. Since the in-queue task repeats, the caller would have a chance every minute to go to voicemail.
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Melissa Bailey
Genesys - Employees
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