A call center is an example or a type of a BPO (Business Process Outsourcing) unit.
Call centers are those where telephone calls are being handled that may be inbound (called by customer for getting information about a product) or outbound (employees of center call people to sell some product or to provide information about some service) whereas BPOs are those where any type of outsourcing work is done. A BPO may be doing HR outsourcing job for various companies, handling accounts for different companies etc.
Hope that you have got some help in differentiating BPO and a call center.
From United States
outsourcing is something as familiar as your part time maid or CA filing annual returns " one tends to outsource non core jobs on price factor to others".
COPC ver 3.3 gold doesn't make any such differentiation in proper nouns except where they add tangible value.
From India, Delhi