There are extensive tables showing grades of service for various call volumes and quantities of equipment. This situation might occur during a natural disaster, such as a tornado hit. Now let's look at the choice of busy hour and busy day. The traffic engineer needs to pick the most likely busy hour and estimate the number of CCS that will be offered during it to each pool of common equipment. Skill, training and experience still come into play because the traffic engineer needs to understand factors that might change the estimate of the network's load.
Data calls over a telephony modem are a good example of such factors.
As I mentioned earlier, concentration has a long history. I am indebted to Dan Paone, director of product marketing for Antec, for his excellent white paper on this history. Much of the flow of thought that follows comes from reading his document. This device recognized an off-hook condition and provided the connection to a dial tone generator. In the first 100 years of telephony, this method for sharing common equipment needed to provide telephone service permeated into every type of telecommunications switch.