It is the business practice of sending all clients' billing info to a separate company, who then does everything for you. It also allows a company to cut down on time and human resources spent on administrative tasks.
Call center - is a center where the company's incoming and outgoing calls are taken care of, it can be a centralized office, used for receiving and forwarding high volumes of information, incoming by the way of phone calls. There are two types of call centers internal call center and outsourced call center.