What are SIM cards?

Anyone who uses a mobile phone will be aware that slotted in securely to the back of their handset is their own unique SIM card. This little card holds some unique information about the phone user, including their cell phone number, and details of their contacts. The first SIM was produced in 1991 by a card making company in Munich.

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 the unique details of each phone user are stored on their SIM card and not their mobile, this means that a user can simply remove their SIM and transfer it to any other phone if they want to use it.

When you swap your SIM card to another mobile, not only does this allow you to operate this new phone, but it also transfers your mobile telephone number to your new handset, as well as your existing phone contacts and saved text messages. This is because all of these details are stored on your unique SIM card.

Many phone users will be aware that their phone is locked. This means that their phone will only work with the SIM card issued to them by their particular network. Mobile telephone handsets bought SIM-free are not locked and can be operated with any SIM, but contract phones are always locked.

This is as a means of protecting the mobile phone networks who often offer handsets that would cost hundreds of pounds SIM-free, free of charge to their contract customers. They want to make sure that the customer and their phone are locked to their network for the entirety of their contract so that the network is sure to recover the costs of subsidising the phone handset.

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.

These SIM-only offers allow travellers to benefit from far lower international call costs, and keep in touch much more economically while on holiday.

About the Author:
Vodafone offer a huge range of phones and price plans. If you like smart phones, you will love the Samsung Jet with its fast internet for express browsing and downloads.