Before mobile phones reached India and landlines were too expensive for the masses many shops offered STD & ISD. STD is short for Subscriber Trunk Dialing or when you dial any number within India, without operator assistance. ISD is International Subscriber Dialing, outside of India. STD calls were available at most times ISD had to be booked in advance.
I remember trying to book an ISD call in 1976 to call my parents and wish them Happy Christmas, but after a couple hours of crackling lines, talking loud to the operator, repeating the number, getting cut off and trying again..............I gave up. Walked to the Post Office and sent a letter. It was probably faster. I tried again 3 years later, I wanted to tell them I was pregnant and they would be grandparents by the end of the year, but that didn't work either, this time I didn't write a letter but decided to keep the secret until I arrived home 7 months pregnant.
Yes it was a surprise.;)