MTS Bill issue

I took MTS from corporate office with 650/- plan. at the time of purchasing i paid Rs.2500/- and there was no bill for three months. the seller told to me that after 3 months you will get bill according to rs.650/- but now i am getting bills according to Rs.750/-plan moreover last month i paid my bill before due date but this month they added Rs.100/- stating that it is fine for late payment. MTS is providing very bad service. if i write mails to customer care then they are not responding moreover no response over calls also.