When making the first telephone call to a new customer, it is often difficult to manage your own prejudices. For instance, you know statements, and follow-up invoice copies have been sent, and there has been no response from the customer.
Based on previous experiences, you may an inbuilt prejudice against this new customer and think "Here we again - another dud customer!"
If you go into your first conversation with this thought, you may affect the possibility of obtaining the truth of the situation as to why you haven't been paid.
Therefore before making the first telephone contact, train yourself to be calm, reasonable and put yourself into a friendly enquiry type mood. Do this, and you may well receive a reasonable response as to why you haven't been paid yet.
Take the angry "it's their fault" approach and you may alienate the customer forever. There are no winners in this situation.