This is an issue of simcard.Try to use another simcard.

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Have you tried purchasing another SIM card to see if that will fix the issue? The SIM cards usually don't cost much, and can be purchased online.

I bought this phone a couple weeks ago. It is a gsm unlocked phone. I have att as my carrier. My Problem is every few days it will say no service and the only way I can fix it is to take out my sims card and put it in my old phone for just a second rhen put back in my new phone. I have tried just taking it out, and putting back in and not putting it in my old phone, but that does not work. I have had to do this now 5 or 6 times although it works every time it is annoying that I have to carry 2 phones. Please help if anyone knows how to fix this!!!