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.