A company which deals with customer service with the help of telephone calls which are undertaken by Telephone Customer Advisors (Consultants, Agents). When consultants call clients, we call these outgoing calls. In this way we can present offers to the customer or allow them to change their services from another provider (gains), help them to extend their contract under better conditions (maintainance) or offer different services especially chosen for a certain client (upsell). On the other hand, calls which are made by the client are called incoming calls. Among other things these may be calls to the telephone information line, complaints services, helpdesk, back office, consumer research. Call centers which are developing their services further are becoming companies such as contact centers.