[Gta04-owner] Antwort: Re: Amazing adventures with a brandnew GTA04...

Thomas Gstädtner thomas at gstaedtner.net
Sat Feb 11 15:18:50 CET 2012

On Sat, Feb 11, 2012 at 12:41, Flemming Richter Mikkelsen
<quatrox at gmail.com> wrote:
> Maybe a little OT, but interesting.
> I have played with SIM cards a little.
> I had the sim in one phone (phone A), registered to the network.
> Then I moved the sim to phone B and registered it to the network.
> Then I called the sim from a landline and both phones started ringing.
> This only works for a brief moment, but, in theory, could mean that
> the sim are only needed for registering.
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Yes, SIMs are only used for identification, as the name suggests.
In theory you don't need a SIM at all, and in many countries the
providers allow phones to be used without SIMs - usually for emergency
calls only.
As providers usually allow you to use any device, they need the SIM
registration to make sure they charge the correct party.
The behavior that you can switch and it will work for a moment is
unusual and likely modem and provider dependent.
Checking the SIM is the modems job, and if it does not notice the SIM
is missing and cache the relevant data on the SIM, there is no reason
why it would not work for a while. The network itself does not care,
the modem identified you and it is its job to answer.

It would be Interesting if the behavior is the same if you don't
switch from A to B but from B to A instead.

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