The workload of your system is 16 calls per 20mins, equals 48 calls per hour. So you have to use at least 48 agents to prevent the queue from overloading. (As you will remember, erlangc-model implies that noone will hang up during a wait.) On the other hand, calls come up in an unsteady process.16 agents DONT provide 100% SLA! 100+ agents are required, because of the (theoretical!) long waiting times that may occur. 90% in 15 seconds is causing this huge amount of agents available.