Whilst I do not profess to be correct I think my answer is closer to to the truth than the previous answer:
Service level is the % of calls answered in a given time frame. For example if you had a service level objective of 80% of calls answered in 20 seconds then the measure of that is the number of calls that were answered in 20 seconds. If all calls were answered within 20 seconds then you have a service level of 100%.
The time frame agreed upon is the key to service level measurmenet and management.