Airtel — no signal problem

I was not getting any network. After trying the sim in different phones i realized problem is with sim card. I Called the customer care at 121, they advised me to change sim. I got the new sim and inserted it in the phone slot. I received the signal in 10 min and a msg stating "your services have been set. Please restart handset". After restarting i'm getting a "no service" message. What is the issue. Why do i have to keep running around repeatedly.