What cause my iPhone 3gs to not unlock?

What cause my iPhone 3gs to not unlock?

If you are not unlocked your iPhone 3gs, you can't use it to other networks in different countries and in your country. For example: Consider you have two iPhones. One is with AT&T and another one is with T-mobile. If it is an unlocked iPhone, you haven't face any problem. But, if you have a locked iPhone, you can't interchange sim cards from your one iPhone to another. In that situation, you need to unlock your iPhone from the locked network. Once, you have unlocked your iPhone thoroughly, you can use it with any other GSM networks independently.

If you unlock your iPhone 3GS from your network provider /from any of the online vendors like http://www.onlinegsmunlock.com/apple-iphone-3gs/rs16wp9/( who provides remote unlocking service in online ) , you can able to swap from your current sim to any other GSM sim cards.