Cell phone water damaged after handed in for repai

I have a contract cell phone with mtn, the phone was handed in and collected on the 17th January after complaining that the battery didnt hold a charge. They had replaced the battery but the keys now did not all work after collection. On handing the phone in the 2nd time they checked the water damage indicator on the battery and it showed there was none. They now say the indicator on the key pad shows water damage. I dont see how i should be made pay for this when thew phone came back after being repaired and had this issue but was never there prior to handing it in. Also if this was the case why was water damage then not reported the first time thew phone was handed in? Kindly help me resolve this issue.