If you have problem in accessing with any specific SIM card in your iPhone, You can make a complaint to your network provider and get a new one.
If you are not able to access any SIM card in your iPhone, Take your iPhone to a local mobile store.
If you are not able to access any SIM card other than a specific SIM card in your iPhone, then your iPhone must be network restricted. You need to unlock it.
The safest and easiest way to unlock an iPhone is to unlock through remote unlocking method. You can get the remote unlocking for an iPhone from www.wickedunlock.com/apple/rs11wp8/
at affordable cost. This method of unlocking doesn't require a Jailbreaking too.
Here are the steps to unlock an iPhone,
1. Wait for us to send you an email saying your phone has been activated or remotely unlocked.
2. Install the latest version of iTunes.
3. Make sure your Apple iPhone phone software is updated to the latest version using the iTunes software.
4. Connect your Apple iPhone to iTunes with a NOT ACCEPTED
5. Wait until iTunes detects your Apple iPhone.
6. Disconnect your Apple iPhone and reconnect it after 10 seconds.
7. Your Apple iPhone is now unlocked.