Ring your mum.

In the 1980s I did teletraffic design in regional Queensland- I decided the number of connections between and within telephone exchanges. This determined the number of calls that could be made at once.
For a small 90 line exchange this might be 4 internal calls, six outgoing calls and five incoming calls at one time.
The numbers were determined by measurements of the number of calls and mathematical analysis. The Erlang Loss formula which is recursive and probabilistic was the main tool. In the early days of personal computers I wrote a program to calculate the Erlang loss using the Pascal language.
I would be considering if a single telephone exchange should be able to make seven or eight outgoing calls at one time.

The units of measurement were Erlangs - One Erlang was one current call.

The busiest day of the year for phone calls was Mothers' Day. Everyone phoned their mum even though calls were expensive and if the network was congested they would try again.