Question

Full form of STD,ISD and PCO.where are these used

Answer 1

STD.- Subscriber Trunk Dialing

ISD.-international subscriber dialing

PCO.-Public Call Office

They all are used from which you can call somebody either in your country or anywhere in world.........

Answer 2

STD-Subscriber Trunk Dialing (or) Subscriber Toll Dialing
ISD-International Subscriber Dialing
PCO-Public Call Office
These are called Telephone Systems