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I was pointed here from WorldBuilding.

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. I want to be as accurate as possible, and I don't know enough about cellular networking.

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

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  • \$\begingroup\$ You'd be dialing your own number right? In that case you would just access your own voice mail, or get a busy signal if you didn't have it. \$\endgroup\$ – DKNguyen Feb 21 '19 at 19:55
  • \$\begingroup\$ The system would probably throw a fit and kick both of them off the net. \$\endgroup\$ – JRE Feb 21 '19 at 19:57
  • \$\begingroup\$ @Toor nope, that's layers and layers higher up the stack than your device would be able to access. \$\endgroup\$ – Marcus Müller Feb 21 '19 at 20:01
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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.

Normally, your SIM card would be impossible to clone (it contains a secret that can't be read, but is used by the processor in the SIM card to en- and decrypt specific things, which doesn't allow you to "get" that secret). But, of course, the entity that produces the SIM card might choose to produce two identical ones. Or, you know, time travel.

Anyway, the network would detect that two phones with identical subscriber identities try to log on the network. Something similar can happen when your phone wanders from one cell tower to the next, when something on the network has gone missing, so that the first tower didn't notice you were gone.

So, I'd expect the network to "ask" the phone that was already logged into the network to verify it's still there. It will be. The new one will still try to log on.

The network will throw out one, or the other, or both or at the very least not allow both to make calls / use data at the same time.

How to handle that situation is completely up to the network, but you can be certain that nothing like "both phones ring at once", "both get the same text messages" or "when one makes a call, the other can listen in" happens: that's simply not how these networks work; there's central databases that match SIM cards to subscribers.

Nothing exciting.

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  • \$\begingroup\$ It wouldn't even allow calls to emergency services? \$\endgroup\$ – zeph Feb 21 '19 at 20:01
  • \$\begingroup\$ @zeph that depends on the network operator; some legislations require that even phones without SIM cards can make emergency calls, others require that only phones with valid SIM cards identifying them can. Since this "problem" wouldn't arise in the real world, it's uncertain how the networks would react. If you're in a country where you can place emergency calls without a SIM card, removing the SIM card should probably allow your protagonist(s) to make emergency calls. \$\endgroup\$ – Marcus Müller Feb 21 '19 at 20:04
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    \$\begingroup\$ @CapnJJ I do study IT networking at uni, but that covers ethernet, not cellular networking. This situation would be the same as packets originating from the same IP and MAC address, but again, I don't know about the cell network. \$\endgroup\$ – zeph Feb 21 '19 at 20:42
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    \$\begingroup\$ @CapnJJ the OSI layer model doesn't apply to auth, so it's not really relevant here at all. \$\endgroup\$ – Marcus Müller Feb 21 '19 at 20:50
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    \$\begingroup\$ @CapnJJ a telephone number is not like a network address. It's up to one instance, your operator to do whatever with a call designated for that. The data networking, e.g. how the voice data reaches your phone, is basically a different plane, completely decoupled from the phone number business. \$\endgroup\$ – Marcus Müller Feb 21 '19 at 20:52

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