You won't know which calls are going to teach you something about your agents or your business until after the call has taken place. For this you need to choose a system that gives you 100% recording and allows you to monitor a 'pool of calls' which are selected through pre-determined business rules. This means a contact centre manager can actively manage the evaluation process by selecting calls retrospectively rather than in advance (which may or may not be of use to the business). This way you can pinpoint a call where say, a sale is lost and analyse why this has happened and how to avoid customer churn in future. Only through recording all calls can you recall and analyse the ones that are of importance to the business.