Benefits of SIM Cards

If you’re a cell phone user whose carrier uses the GSM standard, such as T-Mobile, AT&T, Vodafone, and others, your phone includes a tiny electronic card that offers a lot of value. The card is called a SIM (Subscriber Identity Module) card, and it can maintain your preferences. Just as important, this removable smart card can secure your data, which makes it much easier to move from one phone carrier to another.

Part of the problem with cell phones is that they are complicated compared to a regular landline phone. As a result, few people take advantage of all the features. One area people overlook is using a SIM card. I’ve known friends who have recycled or sold cell phones without removing these cards.

What Are SIM Cards

All GSM phones need a SIM card to run. Some of the newer phones that use a 4G LTE network also need the card even though they are on a CDMA system. A SIM card is a tiny printed circuit board that identifies you to the phone network. Each carrier has its own SIM cards, which can vary in size based on the phone.

Your cell phone may come with one, or you can buy them from retailers. The picture below shows an example of a card I bought years ago from the old Cingular network. The red-circled area is the actual SIM card that you punch out.

Example of SIM card.

What Data Do SIM Cards Hold

SIM cards vary in the amount of data they can hold. Current cards hold between 16 and 64K. In comparison with an SD card that may not sound like much, but it is enough to hold your:

  • cell phone number
  • rate plan
  • LAI (Location Area Identity)
  • Service features and preferences
  • Text messages
  • Address book (info for about 250 contacts)
  • PIN lock

Portability and SIM Locking

As you can see from the list, it is not your cell phone that knows your cell number, but your SIM card. The SIM card is used to authenticate you to your GSM carrier. This means you can remove your SIM card from your cell phone, place it into another GSM phone, and use it. Conversely, if you travel overseas, you might pop in a SIM card for another carrier serving that country into your phone.

While SIM cards are designed to be portable, there are situations where a SIM card won’t work in another GSM phone. More to the point, the cell carrier won’t accept the card because of SIM locking. This is a practice the carriers use when subsidizing your phone’s purchase. For example, Carrier A offers you a $600 Smartphone for $200 with a 2-year contract. If you were to place your existing SIM card from Carrier B into the phone, you might see a message such as Enter Subsidy Password or Enter Special Code. A common example in the US is the locked ATT iPhone.

The practice of SIM locking is more common in the United States. Each country and carrier has its own rules. Some carriers will unlock the phone for you after a set period. This restrictive practice has led many buyers to choose unlocked cell phones which sell for a premium.

Protecting Your Data with a PIN Lock

Most GSM phones offer the option for people to save contact information to the phone or to the SIM card. Saving your addresses to the card has advantages. First, staying with the same carrier means you don’t have to re-key your contact phone numbers. Even if you change carriers, you can get a SIM card reader and software that allows you to manage the data.

Another advantage of saving contact information to the SIM card is you can protect your data using a PIN lock. Many phones have a setting for a phone lock. A phone lock protects someone from using the phone. But, if your phone was lost or stolen, someone could remove and use the SIM card unless it is locked. This is an added layer of protection.

Each SIM card comes with a default PIN that the carrier or manufacturer sets. Sometimes the default PIN is in the documentation or set during activation. If you have enabled the SIM Lock feature and don’t know the PIN, don’t guess. Most cards allow three mistakes and then require you to enter a PUK code. The phone is inoperable until you enter the code for the PUK (personal unblocking key). Your carrier can tell you the correct PUK code to unlock your card after they’ve verified you’re the account holder.

Changing the PIN on your SIM card

If you know your PIN code, you can change it using this string,

**04*old PIN*new PIN* new PIN #

For example, if my old PIN was 12345 and I wanted to change it to 56345, I would type:


Recording your IMEI Number

Another precaution you might take is to record your IMEI number. This is the 15-digit International Mobile Equipment Identity number for your phone. This is a number assigned to each phone that you can report to your carrier if the phone is lost. This would prevent the phone from using the network. The number is usually stamped underneath the battery area. Most phones also allow you to get the number by entering this string on your phone: *#06#

Some phones will show your IMEI number in the phone’s Settings app under About. This is usually easier than trying to find the number on the phone, as it may require you to remove the battery.

Alternatively, some Android phones may show on your Google settings page.

  1. Log into your Google settings  page
  2. Scroll down to the Android section
  3. Click the small triangle to the left of the green Android icon
  4. The panel should expand and show you your phone details. The IMEI number is listed as MEID.

The rules vary about unlocking phones so they can be used with any GSM carrier, but changing a phone’s IMEI number is illegal. You can enter this number to find out details of your phone, such as brand and model. This can be useful information if you’re buying a used phone and the brand and model display as something different.

Although SIM cards are tiny, you can see they offer some big features in terms of portability and security. Instead of worrying about your data if your phone gets lost, it may make sense to protect your investment now. And if you plan on selling or recycling your phone, make sure you remove the SIM card.