Cellphone providers use sim lock, network lock or subsidy lock to restrict the use of your GSM phones to their own network, regardless of the fact that the phone belongs to you.

This lock restricts you from changing sim cards or chips.

You may want to change your network for a variety of reasons, for example:
To use an international sim card when roaming. This can save a great deal of money if you want to call locally or use data services while you are in a different country.

• To change providers: If you aren’t happy with the service you are receiving or simply want a better deal, this lock stops you from changing your service.
• To sell your phone: If you want to sell your phone, having it unlocked can increase the retails value significantly, a definite positive for you!