I.S.D : International Subscriber Dialling (ISD) is the term used to describe an international telephone call dialled by the caller rather than going via an operator.
S.T.D : Subscriber trunk dialling (STD) (also known as subscriber toll dialling) is an term for allowing subscribers to dial trunk calls without operator assistance. This term however, is still widely prevalent in India to describe any national call made other than one’s local unit.In India the term still applies and to dial out of one’s unit area, the relevant city code has to be prefixed to the local number. This prefix starts with a 0. Certain numbers are also prefixed with 95 which are local calls within 500 km.
P.C.O :Public call office (PCO) is term used for the telephone facilities located at public places in India. There are both the versions in service -manned and coin operated.
Yes these are telecom related unless STD could also mean Sexually Transmitted disease