GSM phones are not tied to your or your phone . Instead, it has a removable account card, called a SIM subscriber information module and this SIM has a unique serial number on it. This is what identifies you to your carrier

Normally cell phone service works so that the unique serial number in your mobile phone its ESN is used by the cell phone service provider to identify you. The wireless service provider ‘knows’ that a particular mobile phone’s serial number is yours and knows to send telepohne calls to that phone when someone dials your cell phone.

The important thing about this is that you can put your SIM into any compatible mobile phonme and just asbout all gsm phones are compatible and that phone will now behave as your cell phone, because your carrier sees your sim nside the phone. They couldlnt care less that you’ve transferred phones, all they care about is where your sim card is
And now for the really thrilling part of this. Just as, in theory, you can transfer your SIM car dot any phone, the opposite is also true. Any phone transferab;e with any SIM card. So, if you’ve already bought a luxurious cellular phone that you like and have leanred the features, if you transfer from one GSM carrier to a differnt one, you don’t have to buy a new phone. You can keep the phone you’ve grown accustmed to, and all you need to do is swap the first company’s SIM with your new service provider’s SIM.

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What is a SIM Card @ Articlebase