Airtel — automatic network detection not working in san jose (usa)

My pre paid ph auto deducted local network in hong kong. But when i landed in san jose, usa, it fails to detcct automatically the local network namely at&t. Pl advise corrective action
Earlier, after my first lap of flight to hong kong, my phone auto detected the local network and i used the phone for communication.
But once i reached usa (Saan jose) the local network at&t is not detected. I also enhanced my re charging by another rs250/- still the ph does not work. Sim removed and re positioned once. Still no effect. I get "no service" display. Pl help me resolve problem. Thank you,
Ramasubramanian