What is the best way to calculate the Call Center Service Level ?

The formula

Service level = [ Number of calls answered within the service level threshold] / [Number of calls offered] * 100

Call Center Service Level is actually total number of calls answered within specified threshold time divided by the total number of calls answered and abandoned.

Good AHT will help in improving the SLA.

The Number of Answered calls within the specified time. 

Boost inbound and outbound customer interaction

Total number of calls answered/Total number of calls offered*100 gives Service Level %.

Briefly, The best way to calculate the call center service level is 

1- AHT ( Average Handling time per call ) 

2- AWT ( Average Waiting time on queue ) 

3- ATT ( Average Talk Time per call ) 

4- ACW ( After Call Work ) 

5- Quality 

6- Agent Availability 

Best way to calculated the call center service level is time for which a call was on hold, Number of calls answered to call drop ratio. 

Mystree call agent, who will give you the quality of service that customer recieves

collect data based on the number of calls that had a service level event within your predefined time period: The call is answered. The call is abandoned