Unsatisfactory services

Purchased a moblie device on contract and from the time I got the device I've had endless problem's it's been sent back 4 times already between long waiting periods n the device still having the same problem and also it's very hard to get hold of cell direct call center. The devices had been repaired 3 times already on the 3rd repair I requested the device be replaced but they refused this is the fourth time it's been sent back with the same problem .