Call Center :  A place where calls are being transacted, exchanged, placed, or received, in high volume for the purpose of sales, marketing, customer service, telemarketing, technical support, or other specialized business activity.  One early definition described a call center as a place of doing business by phone that combined a centralized database with an sequencer or ACD system.  However, that is close, but its more than that:

Huge Telemarketing Ops Center;

Help desk, technical support center, customer service center, both internal and external;

Outsourcers, that use their large capacity to serve lots of companies

Order taking center.