Why won't my SIM work when inserted to Lenovo S60?

I have my s60 lenovo for almost a year now, last week I stopped getting signal so I tried to insert another sim card and it has a signal. So to confirm if my sim card was defective I tried it on other mobile phone and yet it has a signal. My sim card is not working only if its inserted to lenovo s60. What do i do? Please help!

Hi Sarah. Is your Lenovo S60 network unlocked? 

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