Most models us a form of erlang; including merlang. Erlang doesn’t consider calls abandoned in queue; amongst other things, and has a tendency to overstate staffing requirements. I would recommend that you look at the HillsB model if you’re looking for more of a do-it-yourself method (http://www.htlt.com/). Otherwise, I would recommend that you seek out a workforce management application; like Blue Pumpkin, IEX, or Rightforce.
Hope this helps. If you want to talk more feel free to email me.
Best of luck,