The call buffer is a protective measure used to filter out calls that are of low quality so that they do not become billable to the receiving agent.
It is the duration of time an inbound call must be connected before it can be considered billable. For example, in the case of TV inbounds for the CCL (Canada Care Life) campaign, all calls that last 14 seconds or less (15 second buffer) will not be considered a billable or "converted" call as it will be assumed that they were unworkable.
In general, the higher the call quality, the shorter the buffer. TV inbounds usually have 15 second buffers while digital inbounds and live transfers have 120 second buffers.