Warranty Repairs

We had a reception problem with the phone and lost connection while making calls even with full signal. After about a month we took the phone to them and they told us that there is nothing wrong. Another three months went by and the problem got worse and we took it in and told them there is a problem with the phone and they must send it in to be checked out. The phone is now four months old. After they have replaced some electric part we had to pay for that part. There were no visible damage to the phone, therefore they cannot tell us that we neglected the phone. This is totally wrong to pay for something that's still under warranty.