Unable to activate my iPhone 3gs after upgrade

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Hello buddies !

I'm new in the forum, and I've a problem. I bought an iPhone 3gs from a friend two days ago, and appearantly it were already jailbreaked.

Yesterday night I tried to make the reset of the phone, and as I was not able to do it (because I didn't had the reset password of the phone) I decided to make the upgrade of the phone from iOS 4 to iOS 6.1.5 and to delete all data of the phone.

The download finished, the new iOS installed automatically in the phone, and the phone restarted. But after the restart the phone it asked for the activation and it wad not able to reconize SIM cards anymore. And after that, I filled all informations asked but no activation.

The phone just show me a message telling that the activation is not possible, Apple's server is not reachable.

Can somebody help me please ?

Thank you in advance !