Clive,
it is possible.
what you are talking about is 'overflow' so no one pickups up on the original huntgroup, or is logged in, and thus the call needs to go somewhere else.
So in CUCM 10.x or 11 you can either decided to Queue in a huntpilot or not. whatever you choose you can forward and unanswered call to another destination. you could divert it to for instance to a mobile number or another huntgroup, or a translation pattern.
but its definitely possible.,
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