Default (Help needed) Mobile signals and no service

Hello all. I got my Xperia z on Saturday 28th and I got home from school yesterday and found I had no service nor barly any service. If I put my sim into my brother's phone it works perfect. And also I have checked the imei and it's not blocked

Help. I don't to waste nearly 300

[ My Devices ]

Some Cheap Phone -> HTC Desire HD -> HTC Sensation XL -> Sony Xperia Z -> Sony Xperia Z1

[ My Work ]

Lightning Speed - HTC Sensation XL