When a client doesn't pay their invoice for a service, Wrigley Paper Products sends a past-due notice after 30 days. If the invoice is still not paid, a second past-due notice is sent at 60 days. Then, if payment is still not received, Wrigley attempts to telephone the client. If the client is unreachable or doesn't begin sending payment within 90 days, Wrigley sends the account to a private debt collection agency. What is this process called? 

a debt collection procedure 

an internal debt collection department 

a payment voucher 

Wrigley's bank statements