GSM - Global System for Mobile Communications.

This is the widly used Cellular network. GSM Net works uses 900Mhz, 1800Mhz for the GSM Phones. In USA and Canda 850Mhz is also in use. There are Dual Band, Tri Band, Quad band phones.
Dual bands come in as 900 and 1800MHz. This is good for Europe, Asia, Australia and Brazil.
Dual band 850 and 1900MHz is Good for North America.
While Tri Band of 900Mhz, 1800Mhz and 1900MHz Gives Good coverage in Europe, Asia, Australia and Brazil and limited coverage in North America. likewise North American Tri Band of 850Mhz, 1800Mhz and 1900MHz Gives Good coverage in America and limited in the resst of the Europe, Asia, Australia and Brazil.

So the Quad - Band will give a world wide coverage. New expensive Mobile phones are coming with the Quad band.
This GSM phones can work up to 35Km, if there are good tall antennas available. Today most of the world uses this system. This is choosen for the digital clearity. This GSM Mobile phones uses a SIM card (Subscriber Identity Module). This card is specially made to keep the owners details and phone book. And the user can use this SIM card on any of his mobile phone. In some cases and in some countries the phone manufactorar can make a SIM lock, thus not allowing all SIM cards to operate on their mobile phones. There can be four factors that will make a SIM lock. Origin of the Countries, Networks, Service providers and type of the SIM. In Some countries SIM cards are locked as it is for originating country only.


Network Lock will allow certin Network only to work.
If you get full SIM card lock, you have to service your mobile phone.
There are laws regarding the SIM locks and Unlocking in most of the countries. The Networks providers can Unlock Any phones which provided by them legally.