What is a SIM Card and Unlocked Cell Phones

by on July 12, 2006

Q: I have been looking into buying a cell phone and have heard a lot about SIM cards. What is a SIM card? Also I have been seeing phones for sale that say they are unlocked. What is an unlocked cell phone?

A: Close to the size of a small postage stamp, a Subscriber Identifier Module, (SIM) Card stores the unique and vital information about your account. When you insert the SIM card into your cell phone, you activate the phone; therefore, this chip may be best understood as the brain or even soul of your cell phone. The SIM card retains such data as your account information, stored names and numbers, text messages, added value features and other network information. Fundamentally, you can transfer your account from one cell phone to the next by simply inserting the SIM card into a different phone because all of the information is stored in the SIM card. It is all about the SIM card.

However, there may be instances where you insert a SIM card into a phone and it does not recognize the SIM card. In this case, the phone may be “locked”. A lock is actually a software setting on a phone that inhibits it from working with a SIM card from another carrier. The phone has been “locked” to keep it loyal to its original carrier. I had cell phone service in Canada with Rogers AT&T and when I moved to the Caribbean, the local SIM card that I purchased was not recognized by my phone so I had to have it unlocked. In essence, a locked phone will only recognize SIM cards from its original carrier. An unlocked phone will recognize any SIM card.