Cisco Unified Enterprise Attendant Console version 8.5.1.578
When an operator has transferred a call but it is returned then they cannot answer any calls (the returned call or any from the incoming queues) until this returned call is cleared.
They have been advised in the past by a former employee to remove the 2 ticks for 'Automatically Campon if busy' in Preferences>Call Transfers, however this issue still seems to be happening intermittently.