Call center A handled 300 calls with a service level of 78% and Call center B handled 1200 calls with a service level of 85%.
Call center A: 300 X .78 = 234
Call center B: 1200 X .85 = 1020
Totals: 1500 = 1254
1254/1500 = .836 (83.6%)
You can also use a weighted average calculation to combine other statistics such as talk time and after call work.
It is important to make sure that the origins of the service level within each call center (telephone systems) are calculating service level in the same way. Calculations for service level may very slightly between different manufacturers. Check the reporting manual for each system supplying data to make sure that the base calculations are the same before combining call center service levels.