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Your best bet would be to have the ATA on its own seat and add it as a presence line + allowing the 10 users to answer calls on behalf of the ATA. If you have a spare seat (such as one used for number porting that isn't in use), I'd change the seat name to be "night ringer", or whatever works best. Otherwise, you'd have to purchase an additional line if no spares are available.

If neither option works, you can make a custom rule on a seat and have the ATA ring for, say, 30 seconds and setup another random device as the second ringing device and still do the above. This would not require you to purchase anything additional and the calls likely should be answered within 30 seconds anyway, so it shouldn't terminate at the second ringing device.