Edwin,
There are three basic pricing models for call centers:
Per minute- this is typically a billing per talk/work minute. time spent in non-productive modes (such as break, coaching, available, unavailable, etc.) is not billed. This pricing strategy has a high risk at low or inconsistent volumes.
Per hour- this is commmon for lower volumes, where all payroll time is billed. Supervisor/trainer/QA time may or may not be billed as well. If it is, it is common to bill this at a lower rate than Agent time. Managers, if billed, are typically a flat rate.
Per call- I do not recommend this, although it is not uncommon. There is a risk based on your handling time. If your AHT is too high, you lose out. If you get it belwo goal, you gain.
I hope this helps.