cellphone faulty

On the 28th September i went to purchase a blackberry z3 at cna hazy-view. they tested the phone and it was working. when i get home it showed that it needed to updated software over WiFi and can not download on the network because is too large to download. the network was in poor state as it showed one bar or looses network for a long time. i thought that this was because the phone needed to update its software which causes the network error. during the period i was on leave so i decided to keep it till i return to work so i could get WiFi and update the software. on the 5th of October i returned to work and immediately downloaded the software. unfortunately the poor network error still appear on the phone. i tried to change sim cards but to no avail. i then thought the phone has a problem so i decided to return it to the store. on the 7th September i returned the phone and been kept waiting till today. i made follow up. I phone them and it seems they cannot resolve my issue. i am told the phone has not returned and cannot give me date when it will return. pleas advise me what i must do. this phone is also under insurance<br>