Other

As well as the Erlang C model there are several other call center staffing models in use.

Simulation

This is the most accurate way of modeling as it simulates thousands of calls arriving using a 'Monte Carlo' simulation. It can also handle a mix of call types and agent skills.

Unfortunately no low-cost call center calculator uses simulation at present. The other disadvantage of simulation is that it is very compute intensive and can be quite slow.

'Merlang'

Merlang (or modified erlang) is an attempt by one vendor to overcome two of the restrictive assumptions of the Erlang C model. It adds another two parameters to the calculations - 'Mean time to abandon' (or MTA) and 'Expected Retry Percent'.

There are several obvious issues with this approach -

Parameter	Notes
Mean Time to Abandon	This is not available from any ACD statistics and must simply be guessed. There is actually no such thing as a 'mean' time to abandon - some callers hang up immediately on reaching the queue; others will hang on virtually forever. The relationship between abandonment rate and time on hold is quite complex and varies very much by the type of caller, the nature of their problem and their expectations. The result is that any estimated MTA probably has an accuracy of no better than 20%. (Actually improving the 'Wrapup Time' parameter has a much greater impact than attempting to include a MTA).
Expected Retry Percent	This is the percentage of callers who, after abandoning, will call again some time later. It can only be guessed at. In addition, retries will generally arrive in a later 'interval' - this is not handled by Merlang.

This model also does not address another Erlang assumption of a 'steady state' situation where all calls are handled in the same interval in which they arrive (i.e. there is never any backlog of calls impacting subsequent time periods).

Our conclusion is that in trying to solve one problem, Merlang actually introduces several others.

Erlang C Free Calculators | Erlang, What Is it? | Erlang calculator, How accurate is it? | Merlang or Simulation?