There are three possibilities available. 1.Check your sim whether it's properly inserted or not 2.There may be a problem in your sim card so try another one. 3.Your mobile locked to particular network provider. If your problem persists,Your mobile phone restricted by some network so try to unlock your mobile phone.You can unlock your mobile phone using network unlock code.It's the best way to unlock your phone.If you need an unlock code for your phone visit http://www.onlinegsmunlock.com/supported-phones/rs7wp7/ I hope it will be very useful to you.
here are three possibilities available to get this error 1.You mobile phone receives weak signal from near tower so try to Swich off your mobile,wait few seconds and switch on. 2.Your mobile locked to particular network provider 3.Check your sim card was properly insert or not. If your problem persists after switch on your mobile phone,definitely your mobile phone restricted by some network so try to unlock your mobile phone.You can unlock your iphone 3GS using Remote unlocking method.It's one of the best way to unlock your iphone without jailbreaking.If you are interested in this method visit http://www.onlinegsmunlock.com/apple-iphone-3gs/rs16wp9/ I hope it will be very useful to you.