My sister just bought W810i, and she's having some problem. She can't receive / make calls & sms. "Network Problem" error was flashed on the screen. She's using Hotlink.

She have to go into the settings and have to search for network only then she can make / receive call & sms. After few minutes, the problem come again.

What should she do? I confirm that it's not network's problem because I've tested her SIM card on my phone and it works fine.