When a collections person first calls a customer about an overdue payment, the general tone of the call tends to be that the customer is avoiding payment, and the collector wants the money - now. This places the collector on the offensive and the customer on the defensive, which is hardly the basis upon which to build a long-term relationship.
In reality, a vast number of payments have not been issued simply because of internal procedural problems, the loss of paperwork, or because a minor issue must be settled. In these cases, the collector is really augmenting the customer's internal systems by replacing missing documents or providing needed information. The tone is entirely different, with the collector initially making the assumption that an error must be corrected, rather than assuming that payment is being actively avoided.
Many collectors have a hard time switching between these two modes, and may pride themselves so much on having an aggressive collection stance that they are unable to make administrative calls. If so, assign administrative calls to one set of employees, and collection calls to another set. Give each group targeted training in their distinct roles, and do not allow them to change roles. By doing so, an administrative caller does not even know how to adopt an aggressive collection tone during a call, and will therefore be much more likely to work with customers to correct their internal procedural issues.
The administrative callers should be assigned the initial customer contact, to see if the payment problem is indeed related to administrative issues. If so, they continue to work with the customer to resolve the problem. If not, they shift the issue to the collections staff, who proceed with the normal range of collection activities in order to receive payment.
In short, the collections role can be split into two functions, where administrative calls address customers' internal payment processes, and collection calls deal with customers' ability to pay.