These details include a unique International Mobile Subscriber Identity, or IMSI, number. The first three digits of which represent the Mobile Country Code, the following two represent the Mobile Network Code, and the last ten digits represent the mobile station identification number.

As well as details used to identify the user, a SIM card will usually also store a certain number of mobile phone contacts and text messages. This way, this useful information will not be lost when you transfer to a new phone, as when you switch, you can bring your SIM card with you.

Your SIM card will store a lot of other useful details too like all of your saved SMS messages and all of your stored phone numbers. The number of each that can be stored depends on the type of SIM card that you have. Early SIMs could only store around 20 contacts and 5 texts, but modern SIMs can typically store almost 300.

In the UK and USA most mobile phones are locked to their SIM card. With a phone that is locked to a SIM you will find that your phone will not work with any other card, and that your SIM will not work with any other phone.

The phones given out by mobile network providers to their pay-monthly customers are usually free, or of a subsidised cost, and so they use SIM-locking as a means of payment security. This way the network can be sure of recouping the cost incurred in offering the phone for free.

Phones can be unlocked however, and this is particularly useful to telephone customers travelling abroad. Call and text charges overseas incur an international roaming rate which makes them far more expensive than domestic calls.

But by unlocking your phone, you can buy a SIM card in your destination country which will offer you call and text rates local to that country, which can save you a lot of money.