The beauty of GSM phones is that it can change service providers at will. There is a SIM card inside every GSM phone, which contains information about your subscription to your provider. Changing the SIM card in your GSM phone lets you keep your same hardware while changing providers. This is particularly appealing when travelling abroad, as prepaid SIM cards can save significantly international roaming charges.
Most large GSM providers "lock" their phones to prevent you from using SIM cards from other providers. Apple has chosen AT&T to be Apple's exclusive carrier partner for iPhone in the United States, thus iPhone will only be available with wireless service from AT&T.
Getting your iPhone unlocked will eliminate this restriction, which allows you to use any SIM card you choose, whether at home or away. And it may be possible to keep your iPhone after changing providers and get all of its functionalities.
Nowadays, many free iPhone unlock software are available on the market like iUnlock. However, unlocking your iPhone also risks. You may break or scratch your iPhone or lose your music and videos in your iPhone, even may be unable to use it with any SIM card anymore.
Lucas, one of my friends did have such problems. He unlocked his iPhone several days ago and had absolutely zero problems. It’s all gravy. The only problem is all the music that was previously on his iPhone is gone. When he connected his iPhone to iTunes, it says “iTunes does not recognize your iPhone, please hit the restore button to restore to factory settings” or whatever it says. Of course he wants the music on his iPhone back. Then he turns to me for help. Unfortunately, it was too late, I had no way.
However, if you want to unlock iPhone but haven’t done that, I have a great idea to backup your iPhone music to computer before unlocking your iPhone. There are lots of such kinds of software now as iPhone file transfer and iPod management. I used a tool called Xilisoft iPod Rip that can transfer my music and videos from iPhone to local for backup before unlocking. After unlocking, I just need to drag local music that was exported just now directly to my iPhone without complicated synchronization.
The app is cool. Just imagine! Transfer and backup your files from iPhone to local, isn’t it amazing? In that way, you don’t need to worry about losing your music unexpectedly due to improper actions anymore.

   thanks,  hope have a nice talk with u