Water damage to my sony Z3

I had my Sony Z3 for not even a week when it got water damage. Sony claims it can take photos under water. and I decided to test that, making sure that all the seals are closed properly.I took the photo, and it stopped working right after I took it out of the water. I immediatly took the phone back yesterday the 2nd of February due to the GUARENTEE and 7 day return warantee. Mtn then told me they will not exchange my phone even though they guarentee that they will replace it, they want to first send it away and then fix it which is not acceptable to me because I feel the phone is only 4 days old and all the seals were still closed. I am very dissapointed about their service and I feel they should stick to their policy of returning the phone within 7 days. I hope that my complaint will be revised and sorted out ASAP. Thank you very much.