PABX means It’s a small exchange where u multplex the lines.Usual combinations for an analog PABx would be 4:16 (means 4 trunk lines,16 extensions), 16:64 so on.
Here U need to switch the voice depending on the traffic. Again there are blocked / unblocked PABx.
In case of unblocked PABx no of trunk and extensions are same.
Normal features of PABx are call transfer,call pick up, conference, callback so on…
Hope this will help