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I was wondering how carriers could track your calls and phone. I know that that there are a few things that are sent to the cell towers (and then to the carrier if it is logging the information) when making a call.

Correct me if I am wrong.

I think that it must must be the number, IMEI (to identify the phone) and location of the cell tower which handles the connection to the phone (other possible data is also the number to which the phone is calling and the date and time of the call).

Are there any more things that are being sent like for instance, phone model SIM card information or something that I am not aware of?

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In general for GSM, SIM card information is used to authenticate the user. Your identifier is the IMSI (international mobile subscriber identity) which identifies you uniquely. It's present on your SIM and is what you can consider your login credentials. The IMEI on the other hand just identifies your mobile phone. You can consider your IMEI like a MAC address of a network card.

There are three algorithms that are used, A3 (authentication), A5 (encryption), A8 (session key generation). Your SIM card will receive a challenge from the celltower to authenticate itself and your SIM card will generate a signed response to proof you are indeed who you are claiming to be.

If you are roaming the challenge and validation of the challenge will actually be done by your own carrier as the A3 and A8 algorithms can be chosen, they are not standardized algorithms. Your carrier will then inform the partner on which you are roaming if you signed the challenge correctly or not.

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Basically, the IMSI and the IMEI are sent to the BTS (cell towers).

After the phone "attached" to the BTS it will only send the IMEI - the IMSI will be replaced with TMSI which will be sent from the BTS and not to it.

In addition, like mentioned above, the phone exchange keys with the BTS in order to encrypt the data (SMS/calls/location info/etc.).

Sometimes, the phone sends some credentials to the BTS. Those credentials can be used to calculate the position of the phone according to the BTS - in order to improve the service.

It's important to mention that the MSISDN is never sent to and from the BTS. In fact, AFAIK most of the phones don't "know" their phone number.

MSISDN=phone number.
IMEI = serial number of the phone (like MAC address of network interface).
IMSI = an ID of the SIM card (like IP address - used for "login" to the network and for determining the MSISDN). TMSI = A temporary ID to replace the IMSI.

BTW - Those are the important stuff.

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  • Yes, the phones don't know nor care about their phone number - it is added down the line by the network. On a decent network you could even have multiple numbers for one SIM or one number shared across multiple SIMs.
    – André Borie
    Commented Mar 7, 2017 at 13:52
  • @AndréBorie Yeah I remembered something like it. Thanks :)
    – Bluestar
    Commented Mar 7, 2017 at 14:00
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The IMEI is set at the factory and follows a pattern. The IMEI number will reflect your phone's manufacturer, model and some of its capabilities. https://imeidata.net/

Note: IMEIs can be spoofed fairly easily. So, you can appear as the same customer with a different device.

The SIM card information sent to the tower is fairly basic. It's a serial number and THAT is tied to your customer records, phone number, etc..

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  • 1
    This doesn't answer the question at all.
    – Lucas Kauffman
    Commented Nov 20, 2016 at 15:59
  • OP is not asking about IMEI or number, OP states that this is being sent - the question is what else is being sent
    – schroeder ♦
    Commented May 15, 2018 at 11:57

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