The link Surrounding customers in the Address/Symbol section opens a list of the 20 closest customers surrounding this customer. The ranking is based on the distance of customers in beelines, starting with the closest customers:
You can use this function, for example, if there is time to squeeze in another call prior to your next fixed call. You can recognize the call urgency through the exclamation points. In case your customers have limited or varied business hours, the option Open now is recommendable because it filters customers that can be called at the current point in time. If you want to further restrict the customer selection, click Open and due now in order to show those customers who are currently due or overdue from the list of all the customers that can be called now.