NOKIA 100 EMERGENCY CALL ONLY

If your SIM Card is not inserted or is not seated properly, it may cause your phone to only allow calls to 911. Ensure your SIM Card is securely inserted into the slot. It may not hurt to remove it and re-seat it. ... You should be able to get a replacement SIM card from your wireless carrier for no charge.

There are two possibilities to get this type of issue.
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 is locked to particular network provider.
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 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 Unlock-Zone.com I hope it will be very useful to you.