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The ins and outs of prepaid SIM cards

Right from the outside, some prepaid cell users might be saying “what the heck is a SIM card?” Legitimate question, actually. Though SIM cards are commonplace for some of us, for others they’re completely foreign. This is because SIMs are not used by CDMA networks — they’re only used by GSM and iDEN networks. For a fuller explanation of GSM and CDMA networks, you can check out an oldie but goodie on Prepaid Reviews.

SIM stands for Subscriber Identity Module, and, well, it’s exactly that. It identifies your subscriber information, allowing you to drop your card into a phone, and voila!, your account is now linked to it. Well, that is, in an ideal world that’s the case.

Restrictiveness of SIM-swapping

It’s not so easy in reality to swap your SIM card. American providers have a practice of locking phones. This means that for the everyday user, a cell phone can only be used on the network for which it was purchased. If you buy and AT&T phone and want to switch to T-Mobile, you can’t just drop your new T-Mobile SIM card into the AT&T phone. You’ve got to buy a new T-Mobile phone, too. That is, unless you unlock your phone.

You could argue that the appeal of SIM cards lies in SIM-swapping. It would certainly hold carriers more accountable — if a subscriber found their service lacking, they could pull out their SIM and drop in one from a new carrier. But the practices of locking phones and of requiring two-year contracts for postpaid service has hampered such measures.

Well, we’re talking about prepaid users, so we’ve gotten past the restrictive two-year contracts. But there is still the issue of locked phones. We’ll get to that in a second.

What’s on a SIM?

I don’t want to get too far along here without mentioning exactly what is stored on your SIM card. Yes, it identifies you on the network, which is its primary focus. This is your IMSI, or International Mobile Subscriber Identity. If you can’t be identified on the network, you can’t get access to it, so this is the most important part. However, SIMs do provide secondary services.

The foremost is contact and other local phone data. Older SIMs hold a very limited number of contacts and messages. However, recent developments have allowed for the expansion of SIM memory. Typically, you can store around 250 contacts on a SIM card, though the capacity to store local messages varies.

Also stored on a SIM is your Local Area Identity, or LAI. This is data sent to the network so they know from where you’re operating. These areas are designated by the operator. When you move to a new location, an new LAI will be written to your SIM card.

SIM card and your prepaid phone

So now that we know some of the information stored on your SIM card, and the intended use of the card, we come to the part where I talk about what your SIM means to your prepaid phone.

The first thing to understand is that not every phone uses a SIM. As mentioned before, only GSM and iDEN networks use SIM cards. And beyond that, the two are not compatible. So you cannot use your Boost Mobile phone or SIM, which uses the iDEN network, with T-Mobile or AT&T, which use GSM networks.

Boost is the only major prepaid carrier, and the only pay-as-you-go phone provider which we review, that uses the iDEN network. What this means to you is that you can’t use your Boost phone on another network, even if you unlock it. It also means that you can’t drop an AT&T or T-Mobile SIM into a Boost phone for use on either of the formers’ networks.

Tracfone and Net10 use spectrum from all of the major carriers, so some of their phones will use SIM cards. T-Mobile and AT&T do, too, as I’ve mentioned. SunCom, too, as they’re now a subsidiary of T-Mobile. So when we’re talking about prepaid SIM cards, we’re talking about these carriers.

Without modification, your SIM will be used to retain your subscriber information, as well as contacts and some other local data, when you change phones with the same company. Once again, dropping a T-Mobile SIM into an AT&T phone will not work.

Of course, you could unlock your phone and change GSM carriers as you please. It might cost you a little extra, but the flexibility you gain cannot be measured in dollars.

SIM warnings

Before I leave this off, I want to make a quick note about changing your SIM card. If you read the user reviews for our featured providers, you’ll notice many problems stem from the switching of SIM cards. Specifically, there are many complaints of users losing their minutes when they insert a new SIM.

It’s tough to ascertain what to do in these situations. Sometimes, a new SIM is required to fix a problem. So how do you ensure that you keep your minutes? My only suggestion is to get the name of everyone you talk to from your carrier’s customer service department, and make sure to speak to a supervisor before you accept any new SIM card. They’re your minutes, and it’s important that you don’t lose them due to some system error.

Any other questions about SIM cards? Ask ‘em in the comments.