Section 451B(3)(b) in Indian Telegraph Rules, 1951
(b)Concessional Charges.- The charges for Speed Trunk Calls [between any two Short Distance Charging Centres of two Long Distance Charging Centres not being situated within a radial distance of 50 kms. shall be reduced by 50% of the charges specified in Cl. (a) above.(i)on all Sundays and National Holidays; and(ii)between 00 hrs, and 05 hrs and 22 hrs and 24 hrs. on other days.