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Say I had a cellphone, let's call it cellphone A, and made a perfect copy of it - SIM card and all - and let's call this one cellphone B.

If I were to call someone on cellphone A, what exactly would happen to both cellphone A and B?

This stems from a short film I'm planning where a character goes back in time with his smartphone, and his younger self - that is now in the same time as him - makes a phone call.

EDIT: This takes place in modern-day Australia, specifically Victoria. Also, the call from cellphone A is to emergency services.

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  • $\begingroup$ Welcome to Worldbuilding, zeph! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$ – Gryphon Feb 21 at 16:26
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    $\begingroup$ How is this about worldbuilding? $\endgroup$ – Renan Feb 21 at 16:39
  • $\begingroup$ @Renan It's worldbuilding due to the context of time travel. We had a long debate a while ago about whether context is sufficient to make a question a worldbuilding question, and the conclusion was "yes." (In particular, homework questions that might be closed by Physics.SE's rules may be open and valid here simply by framing it as part of a built world). That all being said, Electrical Engineering.SE may have a large number of people who know the answer. $\endgroup$ – Cort Ammon Feb 21 at 16:42
  • $\begingroup$ Possible duplicate of worldbuilding.stackexchange.com/questions/101485/… $\endgroup$ – Dubukay Feb 21 at 16:52
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    $\begingroup$ @Renan Real world questions are on-topic, even if they would be better answered on another site. To make a very long story short: it's the price we pay for allowing people to build worlds - any worlds. Zeph, having said that, we also encourage pointing out where you'd get better answers: and you will get better and more complete answers at Electrical Engineering. $\endgroup$ – JBH Feb 21 at 19:46
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  1. At every moment in time the network (= the central computers of the mobile communications operator) has an idea of where a certain SIM is. (Or it has an idea that a certain SIM is nowhere.) This is accomplished by the phone actively broadcasting its identity (SIM + IMEI) to the network and selecting one of the towers which answer. (This is necessary so that the network knows to which tower to send the data packets intended for that SIM.) A phone which has informed the network of its presence and has been accepted for placing and receiving calls is said to be registered on the network.

  2. If you power up two phones with the same SIM, the network will instantly realize that something is fishy. Exacly what the network will do depends on the network; it may be that a second registration with the same SIM will fail; or when the second phone tries to register to the network with the same SIM, the first phone will be deregistered; or they may both be deregistered and the account marked as fraudulent etc.

  3. In any case, at most one of the two phones will be registered on the network, and the other will be put in "emergency calls only" mode. It may happen that they will occasionaly switch between which phone is registered with the network and which is in emergency calls only mode. But, for certain, they will never be registered on the network both at the same time.

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  • $\begingroup$ So then what would happen if an emergency call was made from one of the two phones? $\endgroup$ – zeph Feb 21 at 17:22
  • $\begingroup$ @Zeph: Phones operating in emergency calls only mode are identified by their IMEI (International Mobile Equipment Identity), not SIM card. Such calls are allowed even if the phone does not even have a SIM card installed. Networks are not really prepared to cope with two different physical devices with the same IMEI; some networks can still distinguish between them using a form of radio fingerprinting, others may react by suspending service to both phones, yet others may add the fraudulent IMEI into the shared CEIR blacklist etc. $\endgroup$ – AlexP Feb 21 at 21:27
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Depends on where you are. Much of europe allows cloned sims. The most recently used one will be used for incoming calls.

Apparently in the U.S. this gets the cellular network really confused.

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  • $\begingroup$ I'm actually in Australia, so I would assume that it would function similarly to in Europe $\endgroup$ – zeph Feb 21 at 16:35
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You say " the call from cellphone A is to emergency services."

In that case phone B will never know about it. The call will be transmitted to a local repeater. The repeater will forward the call to the network and that will send the call to the emergency services. A link will be established between you and Emergency.

This happens even if you are sitting next to someone and you call them. The signal does not go from your phone to theirs - it goes all the way through the network.


Note - The following will require checking according to the network

However if emergency calls you back, that's a different matter. Both phones will ring. Only one (the first one to be answered) will be connected. The other when answered will be at the wrong part of the protocol. It will perceive the emergency service to be engaged (which of course it is) and so will not be able to connect.

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  • $\begingroup$ A phone in emergency calls only mode cannot be called. This mode allows placing calls to a pre-defined list of services (e.g., 112 in Europe), but does not allow receiving calls. (Which is reasonable, given that the emergency calls only mode does not even require the phone to have a SIM installed.) $\endgroup$ – AlexP Feb 21 at 21:35
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If both cell phones are the same, the network will accept messages from both phones as they were the same terminal. Depending of the configuration of the network, the second message will be discarded as a repetition or it will be processed as another request from the same phone to register in the network.
But, when a cell phone registers in the network, its cell is registered as well, so when someone calls the phone, the network ask to the core where is the phone. If both phones are in different locations, then the core will only have the information of one of them. Hence, the call will be transfered to the phone with the latest registration.
If your character restarts its phone, he/she will register that phone in the core. Because a restart forces a registration in the network.

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