Bell typically lowballed it, at GTE we usually went a little better on the residential subscribers and a lot higher on the business network end of things.
Now you get into things like CCS and Erlangs.
CCS=Call Centi Seconds. How many seconds an hour can each phone be off hook. There are 60 seconds in a minute and 60 minutes in an hour, therefor there are 3600 seconds or 36 Centi seconds in one hour. To allow each phone to be off hook anytime and never get a busy signal would then require that each phone have a CCS of 36. Bell used to spec a residential CCS of 2.5 on their COs. NEC and Northern Telecom (on their PBXs) went in for a CCS of 5 or a 6. GTE used to spec about a 10 or a 12 for their private networks. I'd have to pull the old priming charts and tables to be sure but those numbers should be close.
At the Bank, the SVP of Telecom told me he wanted 1 more trunk then he really needed. He never wanted people to get a busy signal, but he didn't want to pay too much.
You need to run endless traffic studies, find the "Busy Hour" and then work from there. Before PCs it involved pouring over page after page of green-barred printouts. And after you figure it out you've got to continually monitor it, because - things change!
I actually got written up in a trade journal because I created a PC based application that made the Network Managers job significantly easier.
But that was then.
