Not necessarily a private phone system like in a large office, school, etc.  Any household telephone from the 50's, 60's, and 70's today would be considered a POT.

Nothing digital, think of a totatally pre-computer, pre-internet world.   Remember, like typewriters and carbon paper???

Growing up in São Paulo in the 50's the POT number at home was 7-5574.  How come I remember that but now can never remember my own cell number?

TO PP:  ramal is extension.   To reach someone in an office you dial the main number than ask for extension xyz.  Nowadays of course you can often dial the extension directly from an outside number.  Oh the joys of modern technology...

Be well.

CRS