Karan,
Call arrival pattern does have a significant element of randomness about it and the contrbution of it to non-achievement of SLAs is likely to be greater in smaller centres.
However, accurate tracking of historical data & forecasting should enable you to predict the call arrival pattern. If you are not accurately matching your schedules to your forecast call arrival pattern, this is liable to have a greater impact on SLAs than any inherrent randomness.
Cheers,
Chris