Mobile phone sim cards:

SIM (Subscriber Identification Module): If you need to access the telephone network system, the SIM stores the key used to identify the mobile device (such as a mobile phone or computer) to the network.

 Phone SIM cards contain an individual serial number, the mobile user's unique number, security information (used for authentification to a network), a list of services that user is  to use, and two passwords – one (a PIN) which allows normal use of the mobile and one (PUK) which is used to unlock a mobile phone. The first phone SIM cards were the size of a credit card, but soon smaller phones were developed, and the makers of phone SIM cards were forced to SIMs into a smaller format to fit inside the new style of phones. The new format was called a mini-SIM. These are still the standard size used in most mobile phones today. For modern phones, the SIM is pressed out of the card and usually has a cut-off corner, which ensures that it fits into a phone only with the correct orientation. Today, even smaller SIMs are available for some phones; these are called micro-SIMs or 3FF cards, set in a larger carrier card with older style phones.

Phone SIM cards can be easily removed from your phone so that you can change networks, or mobile numbers, when you wish to do so, though you may need to unlock the phone it before you can install and operate a new phone SIM card.Many of today's suppliers offer free SIM cards, which you can top up with PAYG (pay-as-you-go) credit, and this allows users to swap networks and take advantage of the best deals according to the type of calls and texts they want to send, services they require, and which network they wish to use (including international networks). Providers of some phones ‘lock' their devices so that they can only be used with one network provider, thus making it difficult to change to another network's free SIM card, but it is usually relatively easy to unlock the phone.

If you want to take advantage of a free SIM card offer, you will need to check to see if your mobile phone is locked, in which case you will only be able to use the network operator with which you originally signed up, and inserting a new SIM card will prompt a message warning that it cannot be used with that phone. To find out how to unlock your phone so that you can use free SIM cards.