This is a Time of Day telephony traffic graph – I’ve been looking at them for most of my working life. For a normal business day they pretty much always look like this:
This is how business people use telephones on a normal working day.
They generally get into the office and start making calls at about 9am, work steadily up to about midday, then have a spot of lunch. They come back at 2pm and start calling again, then everything starts tailing off about 4pm as people start thinking of home – or beer, or both.
Telephone exchanges have to be built to cope with the traffic at the busiest hour of the day so since the very earliest days of telecommunications telephone companies have been trying to reduce the height of those peaks and spread the load more evenly.
A call at a peaks adds a cost but a call either side of a peak adds a profit.
As you can see, the network is doing practically nothing after 6pm