The Combination of Confidence and Language should be present while talking to the customer regrading his debt, as per industry requirements now day's people don't have to speak or show any signs of American or Australian accent in their conversation, people look for neutral language i.e. SIMPLE ENGLISH which can understand clearly by anyone.
So its very necessary to be very clear and precised what you talk during the conversation, as you have the very essential and vital data of the customer, any exclusive talks with your customer which is not guided by the company policy can create great mishaps also you carry the responsibility of the process (The Business) even an small mistake can escalate to sad ends for the organization and the bread butter of the employers.
Its very clear when you start taking the calls or when you are ready to called as collector, you are under the guidance of Quality and Operational Managers so that everything comes to your way you can master those as different types of customers, situation and scenarios, and to overcome the situation, if you cannot handle any situation try to seek the help of your managers but don't try to expertise at that moment.