Hi Karthic,
There are all generic certifications and not specific to Call centers.
Six sigma is not acertification its a practice of benchmarking and improvement.
ISO is a certification and its for all any industry.
COPC is a certification and applies to ANY service industry.
The only call center certification is offered by a US company STI Knowledge and is called CORE-2000.
You can search their sites to know more.
Regards,
Ajai