need help with my motorola v3 phone

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hi, Just 2 days back my brother brought a motorola v3 phone for me. it was bought from cingular at&t. after i put the sim cad inside and switch it on, it switches off and then switches back on. this process goes on and on till the battery is over. what i dont understand is if i dont keep the sim card, it just shows 'insert card' message and not restarting itself. i dont understand what to do. i cant return the phone as it is another country and if i do i dont know when i will get another phone.