Telecom Thursday: What’s a SIM card (and why do you need one)?

tl;dr–you don’t.

This link is a much better and involved post on this topic.

SIM cards were first installed on mobile phones to identify the payee of mobile phone plan to a carrier’s network and make one’s phone act on their network according to the specifications of their plan. It serves largely the same purpose today.

SIM Application Toolkit (SAT)

SAT evolved because carriers wanted to offer added value to a phone plan for market differentiation. SAT was how users could access a menu of controls and options for their phone to let them do things like read a horoscope or view sports scores.

As mobile phones moved into graphical interfaces, device makers innovated on features similar to but far better than what was possible via SAT in order to differentiate themselves from competitors. This led carriers to stop innovating on adding value to user’s phone via SIM. Instead, carriers doubled down on marketing, device exclusivity and competing on price for consumers.

Current state of SIMs

You don’t need a SIM card to use a phone, but you do need a SIM to use native voice and texting apps on smartphones. This is because Apple and Google have chosen to ‘trust’ carrier-connected (read: SIM) phone numbers more than virtual numbers. To use a virtual phone number currently means you’re using a non-native app.

Your current SIM has these features:

  • Can access cell tower network
  • Knows who you are and
  • Can act on your plan configurations (ie, no service if over X GB or no roaming)
  • Can call 911
  • Dial carrier-specific numbers to access different services related to one’s phone plan (ie *729 on ATT phones to pay a phone bill)

What’s next?

You might be thinking: if a SIM’s level of involvement with the performance of the phone is so low, than why do I have to change my SIM every time I switch carriers? It doesn’t seem like that piece of hardware is that complicated or carrier-specific. Or you might not be thinking that because you’ve never switched carriers since it’s such a pain in the ass process.

It is possible to live in a world without worrying about SIM cards, but only as a theory. Embeddable SIMs are a concept which would allow a user to change carriers without switching out SIMs. There’d have to be an agreed upon process between involved parties to handle a user going from carrier to carrier but also device to device–Apple would have to work with Google in case they’re the losing company to port a number. AT&T would have to agree on how to switch a user to T-Mobile, etc etc.

There are two possible paths that I think are more likely than device makers and carriers coming together to create eSIMs:

We live in a world where people begin to have data only mobile plans and device makers begin to treat virtual phone numbers as first-class citizen on their OS.


We live in a world where Wi-Fi is so readily available to everyone that they no longer need a data plan.

My first example is very likely to occur in the next couple of years–data is only going to get better and cheaper for consumers, particularly VoIP. I believe virtual phone numbers won’t be accepted as a whole entity by any OS–but instead one provider at a time (Listen , DialPad, etc).

I believe CallKit on iOS 10 shows Apple’s interest in supporting VoIP apps and beginning to think about how virtual phone numbers play with their ecosystem.

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