by Kimberly Munsey-Carlton

Created on: January 07, 2011

Carriers like AT&T and T-Mobile are cell phone service providers who work by using  a Subscriber Identification Module card, better known as a SIM card. A SIM is simply a microchip storage card that communicates with the carrier while also storing subscriber data such as contacts and some text messages. A SIM card is issued by a carrier, activated and provides cell service to any GSM (Global System for Mobile Communications: originally from Groupe Spécial Mobile) phone into which it is inserted. In other words the SIM card is the brain of the GSM cell phone.

GSM cell phones can be unlocked to use in other countries, with other GSM carriers and with prepaid accounts. This can save money and also make a well-loved device more versatile. Most importantly, the SIM unlocking feature can provide consumers the ability to buy phones from people or companies other than their carrier. This can save money but it can also open up a whole new world of devices and manufacturers than the current carrier offers.

With a SIM Subsidy Unlock Code the cell phone can be unlocked and will then  recognize a SIM card from any GSM  carrier around the world. However, if the phone is locked it will only recognize a SIM card from that particular carrier because it is a software setting that keeps the device proprietary. This feature is meant to keep customers loyal to a company who offered free or deeply discounted phones known as subsidized devices.

It used to be that SIM unlock codes were only available from obscure and expensive sources; usually an unseen hacker online. Now most carriers, with some restrictions, will agree to unlock the phone upon request, rarely charging a fee. There is some caveats with unlocking a handset, however. Due to proprietary settings sometimes installed in locked handsets, there may be features that don't function correctly with other carriers. Common issues are MMS, which is picture messaging, and accessing the Internet. While no carrier guarantees they can help you with another carrier’s device there is usually work-arounds for the issues.

One known issue with using an unlocked iPhone on T-Mobile’s network (it is currently sold for use only on AT&T’s network) is the loss of 3G. AT&T’s bandwidth for 3G is different than T-Mobile’s bandwidth so there is no access to 3G and users must accept 2G service. The other issue, and this one can be quite costly, is that the nature of the iPhone is to

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