This should work for all Samsung phones (s2, s3, s4, s4, note 2, note 3, note 4, etc. This may perhaps work on other brand phones as well.
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I have listed 6 possible fixes for your phone. Now, of course this wont help all of you but i say at least 50% of you will have your phone back to par. Each of the 6 possible steps is listed below. If none of these work for you your IMEI maybe blacklisted or not matching your phone.

Also, if none of the 6 fixes works for you another possible fix would be to root your phone and along with several other steps that many correct your sim card issue. You can look here:

https://www.youtube.com/watch?v=_Z6zTfe-sC4
or
https://www.youtube.com/watch?v=i7K6RDVRBhs

If any of the 6 fixes works for you please comment and identify which step worked for you. Also subs would be great too. Thanks.

1- go into network settings, click on network mode and select gsm only or cdma only- reboot phone
2- go into network settings, select network operators, then click search now, then select your provider
3- restart your phone from within the device
4- pull battery out while phone is on, then put back in and reboot
5- go into your date and time while connected to wifi, select automatic
6- insert a business card or a piece of paper into the back of your sim as your sim maybe need to be a little more tug inside.
7- I forgot this one. Ejected and reinsert your sim card! Perhaps your card was not inserted correctly.

All subs are appreciated.

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