Answers

2016-01-30T08:57:41+05:30
Subscriber trunk dialling-STD
International Subscriber Dialling-ISD
PublicCall Office-PCO

This is used in a telephone system allowing subscribers to dial trunk calls without operator assistance through trunk lines .
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2016-01-30T09:00:24+05:30
STD stands for subscriber trunk dialing
ISD stands for international subscriber dialing
PCO stands for public call office
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