Cellphone repeatedly taken in for repairs

My daughter has a blackberry cellphone problem and we have taken the cellphone in 4 times to get fixed. Apparently it is a software problem, and when we take the phone in they say the software needs to be updated, the phone is left with them for a week or so and when we receive it back, it works for a short while and then a white screen occurs and she cannot use the phone. <br> It is so inconvenient for us to keep on going into the shop. All we want is for the phone to be fixed, that is why we are on a contractual basis. Please help !!!!!!