Apple's iPhone hasn't made an obvious dent in the market share of either handset competitors or carriers that compete with AT&T. But it has hit those other companies with something else unexpected -- and unwelcome: The iPhone has sharply raised consumer awareness about the issue of locked mobile phones.

Most U.S. carriers profit from consumer ignorance about the locking issue. But the iPhone controversy is changing all that.

Millions of mobile phone users are suddenly talking about phone locking and asking themselves, "Why is my phone locked?" And that's a good thing. It's time consumers demand unlocked cell phones from their carriers.

What's a SIM card?

A SIM card, or Subscriber Identity Module, is a tiny, removable memory chip used in a Global System for Mobile Communications (GSM) phone associated with a specific account and phone number. A SIM card holds a unique identification number (International Mobile Subscriber Identity, or IMSI), a unique phone number, plus potentially your address book, some of your text messages and other information.

Theoretically, you should be able to remove a SIM card, place it into another GSM phone and make a call from your own mobile phone number. Calls made using your SIM card are billed to your account, regardless of whose handset you use to make the call.

But thanks to carriers, it usually doesn't work that way.

Your phone is probably locked

The iPhone is and will be sold through a single carrier in each of the countries where Apple chooses to sell it. In some countries, such as France, locking is illegal. Apple announced this week that French telco Orange will be the exclusive iPhone carrier in France and will sell a locked iPhone, plus a more expensive unlocked iPhone in that country.

This stark fact makes it clear to everyone that locking and unlocking is a mere carrier choice, a deliberate snippet of code whose sole purpose is to limit your freedom and choice so the carrier and handset maker can make more money.

Apple isn't alone in locking phones, either. U.S. carriers vary wildly in the degree to which they lock phones.

Some SIM cards are relatively open, run Java and are based on standards. But many are proprietary "native" cards, which run vendor- or carrier-specific software that deliberately limits their use in some way.

A full SIM card lock ties a SIM card to one specific phone. If you get a new phone, you need to get a new SIM card, too. A "service provider lock" makes phones work only with SIM cards provided by a particular carrier. Other locks can block use abroad or with other kinds of SIM cards.

Locking is easy for the carriers to do. They simply choose the type and degree of locking. And unlocking is easy, too.

If consumers remain ignorant and demand little from their carriers, we'll get locked phones and limited options. But if we demand unlocked phones, we'll force change in the mobile phone experience that benefits everybody.

Locking a phone in order to steeply discount it but prevent it from being sold at a higher price is legitimate. But carriers should offer nondiscounted unlocked phones as an option, and customers should demand them.