The SIM card in your mobile phone is what connects it to the cellular network. It contains all the information needed to work with a mobile phone network: your phone number, its type and frequencies, its country of origin and its serial number, so you can easily track it down if it’s stolen or lost.
A SIM card is also a chip used to identify you to the network. This is not a fully-fledged smartcard; there are no RFID or NFC chips within the SIM card itself.
A SIM card is merely the bearer, the user of the phone. You use the SIM to make calls and send SMS messages, but the network uses the SIM to activate the phone and, ultimately, to deliver you messages and data.
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You can have several SIM cards in a mobile phone, in order to use it internationally or on the same network as your home network. To do this, you need to ask the network provider for a new SIM card and number when you go to a new country or change your plan.
The SIM card for your mobile phone
The SIM card used in a mobile phone is the unique ID for the particular model, made by the phone manufacturer. This ID is often the part of the SIM card that makes the phone different from others in the range of brands.
Your current SIM card (actually the part of it that you use to make calls) will stay with you, even if you return the mobile phone to the shop where you bought it. It’s the SIM card that’s lost or stolen that needs to be replaced.
SIM cards in mobiles and handsets come in two types:
- internal SIM cards, which can be inserted into the SIM slots in mobile phones
- external SIM cards, which can be used by any portable device such as a smart phone or a laptop.
When a mobile phone is bought, you’re given an ‘internal’ SIM card as standard. This ID identifies the phone and can be used to connect to the cellular network. This is the type used by public phones, often just called a ‘mobile phone’.
If you want to make calls or send texts to a mobile number that doesn’t already have an internal SIM card in it, you need to buy a second SIM card, the external SIM card. External SIM cards are available in a range of sizes and have different frequencies.
The various types of SIM card
When you make a call, the network provider updates the information about the SIM card in your phone (called an Account Information Unit, or ACE), which gives you a new ID. The mobile phone uses this ID to connect to the network. First it tries to connect to a mobile phone network it already has access to. If it finds one it usually offers a good deal. It’s either free to make calls or low-cost minutes, or in some cases both.
In some cases, the network may offer the phone without an internal SIM card or at a very low cost. You’ll pay for the mobile phone separately, which will have its own SIM card.
Some mobile phones support multiple SIM cards, in order to be able to use two numbers on the network. In these cases you can keep a phone, and get a cheap pay-as-you-go deal for the mobile phone and another one for your personal number, which can be used as a ‘burner’ SIM for your own personal calls or texts.
Other phones support only one SIM, for use with only one person, while another one connects to the network as a second number that can be used for both personal and business calls.
You can keep your existing mobile phone and SIM card. And then change the SIM you’ve got in it for the SIM that came with the new phone. This is sometimes called a ‘device unlock’.
At the end of the phone contract, you can either buy the phone and SIM from a network provider, or get a pay-as-you-go deal, so that you can change the SIM at any time. This is called an ‘unlocked’ phone. You can sometimes use a mobile number registered to another phone that doesn’t support a mobile network. Like a pay-as-you-go phone with a SIM card from another country. This is called a ‘burner’ phone.