What is a telephone call for the unpaid invoice? Most business people consider it to be a "debt collection" call. Perhaps it can be viewed as such from time to time.
However I believe you should consider the call as a customer service or sales call. Experience shows; by using this approach, you will achieve far more beneficial results. Look at the two following examples.
1 The call for the unpaid invoice might reveal that the customer has not yet received the invoice. The call has now become a customer service call.
2 Whilst speaking with the customer, you find there are problems which have gone unresolved. Now you begin to understand why the customer has not been buying lately.
Resolve the problem, and you start to see the customer buying again. Now the call has become a sales call.
The result of either call was only possible because you reframed the context in your mind, attitude and words about the nature of the call. The telephone call has turned from the dreaded debt collection call in to a customer service or sales call.
Of course, from time to time you will to put your "debt collector's hat" on. However let the customer initiate that response. Always try the customer service or sales call approach first.