In those days, STD calls are counted with units. Four seconds is one unit. and in evening time, from 6 to 9 pm, and morning upto 9 am the units time was double the standard unit - so eight seconds per unit. and from night 9pm to morning 6 am i think, its 16 seconds per unit. and one unit is charged with 50 paise.
And before that, we book trunk calls. no direct dialing. We tell the city and the number and they connect and to whom to talk to at the other end. That exchange will call that other number and then connect us. At the end of the call, they will tell how much was the charge for that call. Monthly bill la yum vandhurum full list of calls and trunk calls list separately. Businesses use to pay 200 to 300 rupees for phone bill. Huuuuuuge money those days.