Extra bill by MTS

Hi,
my MTS dungle no-8182965157… i had purchased this one in bangaluru in october last week,2013.while purchasing this one their agent told me that the cost is Rs400 n its a postpaid plan of rs500 n while paying the bill u got a consesation of rs100 means i have to pay rs400.Then upto two weeks they didnot gave me the connection. I regularly called them then they give me the connection. I used it for one month n finally a got a surprising bill of rs20000.so i had not pay the bill. they called me n told that we gave u a offer that u have to pay rs4000 means 40% of ur bill.I comaplain abt the bill but no body respond me.finally their agent called me n told me that both of us(me n MTS) has done a mistake..as in my postpaid plan i got 2.5gb of 3G n rest unlimited but in 2G speed…but they forgot to put the limit after 2.5gb..n they told me that u had download 30gb.but i had not used that much..when i told them that i am a student how can i afford this much..then didnot listein to me…my question is that if u knw that u have to put a limit after 2.5gb then why they didnot put the limit..Its simple they have to take rs400 frm me but they r not agree with it.Its simple if i m not paying the bill pls disconnect the connection but they r not listing n continous calling me.,Pls help me..