I have purchased duplicate iphone from dubai...now its shows invalid sim and some time it shows my network name but after some time it shows invalid sim card....

Your iPhone says "Invalid Sim" means that your phone is locked to particular Network.To solve this problem, you need to get your iPhone unlocked.You can get your iPhone unlocked from Network lock using Remote unlocking method.Which is the safe and secure method to unlock iPhone.You can get this service from Unlock-Zone.com here you need to specify your country,Network name to which it is locked,IMEI number of your iPhone.And you can get free unlocking free unlocking instructions from MobileUnlockGuide.com using this you can unlock your iphone from Network lock easily.

Your iPhone says "Invalid sim" means that your phone is locked to particular Network.To solve this problem you need to get your iPhone unlocked.You can get your iPhone unlocked safe and securely using Remote unlocking method from SuperUnlockCodes.com here you need to specify your country,Network name to whcih is locked,IMEI number of your iphone.

A simple way to check is by inserting other network's simcards with the iPhone 4 and if it works without any problem then the iPhone is unlocked...if it shows any error messages like "Invalid sim"or "Error sim" then the phone is locked with a particular network.To remove the network lock you can approach Onlinegsmunlock.com and remove the network lock permanently using their remote unlock service.If you have any doubt with what network the mobile is locked too...you can use their contact us form to get the respective details.

Your cellular provider has to have the phone you are using on file so that the network will recognize your sim card. If you have AT&T you can log onto your account and change it yourself. You can also call and simply say it won't recognize my sim card, they should be able to indicate the phone you are using and it should then register on the network.

If you insert Sim it shows Invalid Sim means it locked to some Network,You need to Unlock it,You can get the free unlocking instructions from here
Mobileunlockguide and get the Unlock code from here Superunlockcodes Then you can use any GSM Sim in the Phone,it will not show "INVALID SIM"

If it says Not valid or Invalid Sim Or else Sim error...It may locked to some Network,you need to Unlock it...To Unlock the Mobile,get the code here http://www.theunlockarena.com/nokia-e63/rs13wp9/
and Unlock it.

i have a china made mobile n8 but after inserted it shows invalid sim what's the solution of this problem

you should register the IMEI no. of the sim slot2 with the service provider of your second sim. for example if its an airtel sim, call the customer care and they will tell u when and where to register your IMEI num. probably this should solve your problem.

there is a solution for your problem; go to ebay.co.uk and search for 'invalid sim' the sellers name is 'sim-solution'

The blackberry from the states will be locked to that network. You need to contact the company/provider in the US you bought it from and ask for the unlock procedure.