If you are not buying an iPhone directly from Apple, we suggest you to check the SIM lock status. If your iPhone is locked then that means that you can only use the phone with a single service provider and if the phone is unlocked, then it means that the phone can be used on any network service provider. It is recommended that you buy an iPhone unlocked as it gives you greater freedom in using your phone overseas.
To check if your iPhone is locked or unlocked, you must know the IMEI number of your phone. You can find a free iPhone IMEI checker service at iphoneimei.net. Go to this website and type in the IMEI number to know if your phone is unlocked or not. IMEI number of every single iPhone sold is unique. If your device is locked, you can unlock it easily by using the service from iphoneimei.net. Since they whitelist the IMEI, the user can upgrade or downgrade to any iOS versions without getting the iPhone locked again. Also all iPhones from 3G to 6 Plus are supported running any operating system.
The entire method does not void any warranty so if you have bought an Apple Care plan to extend the phone’s warranty, there is nothing to worry about. No need of fiddling with difficult-to-use jailbreak tools just to get the phone unlocked again.
To find the IMEI number, go to dialer on your phone and key in *#06#. As soon as you key in those characters, a unique sequence of numbers will be displayed. This number should match the one shown on the box of your iPhone. If it does not, then you have bought a stolen iPhone and in such scenario, it makes sense to ask for a refund or replacement from the seller.
Another way to find the IMEI number is from iTunes. Fire up iTunes on your computer and connect your device. Go to the Summary tab to find your phone’s serial number as well as IMEI number. There exists one more method. On your phone, go to settings > general and then to about. You will find both serial and IMEI number there.