The Erlang C traffic model makes some assumptions about the nature of the traffic into a call center (see https://www.erlang.com/support/cc-excel-faq/). They include
– Call arrivals are random
– Service times are exponential
– Callers are answered in the order of arrival
If call arrivals are not random, then Erlang C can tend to underestimate the agents needed. For example, if you have a sudden peak of calls, you’ll need more agents than normal to handle that peak.
If you’d like to send me some example figures, I can check them against our tools and make sure CC-Excel is giving you the expected results. My email address is firstname.lastname@example.org.
All the best,
Westbay Engineers Ltd.