liquid dam not rspnsble 4 charging port failing

in Jan 2015 i upgraded from a use my own phone to a bb 9720 as my prev bb was giving trouble with dropped calls. the dropped calls continued. I was told to get a new sim card which i did. just 2 months later, the new cell charging port failed & under warranty i got a 2nd new phone. 4.5 months later, the charging port on this cell failed too. I took it to midrand & was told that a small dent in the casing would void the replacement warranty & to book it for repair which i did. I did not hear from mtn & after a few calls & emails was told that mtn is waiting for me to pay the money..? what money - the R330 assessment fee as mtn found liquid damage on the cell & the phone cannot be repaired. At no stage did i get this phone into water. The charging port on this model is clearly a problem & the staff at midrand said that usually the port disappears into the phone?? This was not the case, but the phone did not charge. Now i have no phone & no-one will talk to me & i must continue with my contract incl this phone for the next 18 months. I am most unhappy. I wish to revert to use own(old) phone deal, or get my phone repaired by mtn. please assist. I have 3 numbers on account with mtn