my bold 9700 will not get signal anywhere but outside, i put my sim in my mates phone and will get signal but not in mine, i was on vodafone and i thought that was why, but i put an orange sim in and it is just the same, it just says 'sos' all the time, i bought the phone off a friend and it worked fine for them, i change the network settings to orange, but it either goes back to 't mobile' or '3 uk' a couple of minutes later. i dont understand what the problem could be, anybody help me ???

Is the device unlocked?