AIRTEL/NOKIA
Different call charges on the same cell no

Hi, i am a airtel prepaid mobile no. user my complaint is that on my mobile to an other airtel mobile call charges are different 1 mint . to one mnt RS. 1 (withe tarrif voucher) & next call of two minut. is RS. 1 & third call After five minut of 3 minute is Rs. 2 After all having tarrif plan of Rs.69

what are the reason of differrent call chares at the same time?