Hi Alan,
From the quick test I performed, calls will remain in a Queue until answered or until the caller hangs up. If you haven't done this already, one option is to adjust the Queue Timeout settings to see if this allows calls to funnel into your Overflow.
https://docs.8x8.com/8x8WebHelp/VCC/configuration-manager-general/content/channelphonelivrscriptobjects.htmAlso don't forget to ensure your Phone Script is setup to forward to the desired Queue or Agent Group. For instance, if you have callers in a main Queue and there's no actions within the script to direct callers after a certain period of time, they won't go to the Queue you need.
Hope this helps.