Unlocking is the key: then your sim is a trump card

Unlocking is the key: then your sim is a trump card

Switching to a low-cost sim-only deal is relatively straightforward, assuming you follow a few basic principles. If you open the back of your phone, somewhere beside or behind the battery is a small card that looks similar to the chip on your credit or debit card.

The subscriber identity module is essentially a removable smart card that logs whichever phone in which it is placed on to that particular mobile network.

Sim cards can be easily switched between phones - whichever handset yours is in is the one that will ring when someone dials your number.

Unlocking

The sole major complication is that a large number of phones are "locked" to the network that originally sold the phone - and a locked phone will only work with a sim card from that network. If you want to use a card from a rival network, which is perfectly legal, you need to get it unlocked.

To see whether your phone is locked, simply put a friend's sim from another network in your phone - if it works, yours is unlocked.

If not, any number of independent stores and market stalls will do this for you - but it is possible to do it yourself armed with the correct info because they simply change the handset's codes to remove the locking function. This service typically costs around £10.

Anthony Ball, of comparison website Onecompare.com, warns that a small number of handsets cannot be unlocked, so it is probably worth getting yours unlocked.....continued below

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before purchasing the sim card. Phones from the 3 network can be problematic. Once unlocked, your mobile will accept sim cards from any network - you can even put in foreign cards if you're travelling abroad.

Once you've made your choice, log on to that company's website or go to one of its stores and make the purchase. Because these are effectively short-term contracts, users have to be over 18 and face a credit check. Parents will have to front their child's contract.

Your new card will be handed over there and then or posted to you. Once it arrives, simply replace your sim with the new one and start making calls.

Porting your number

If you are very attached to your mobile phone number, there's nothing to stop you keeping it and switching to a sim-only deal. Strangely, this can be easier if you are switching to a rival network.

T-Mobile confirmed this week that its customers moving off an expired contract can keep their number if they move to one of its sim-only deals. If you are staying with the same network, it's worth checking this is possible before you agree to switch. The networks must offer to port your number to a rival - and it does work relatively well.

New phone

If you've decided you need a new handset, going sim-only is still a possibility. Plenty of pay-as-you-go (PAYG) phones offer decent cameras and other services. Don't buy a sim-free phone - these are still expensive. Much better to buy a PAYG phone linked to the network you've chosen. Once you've used up the free airtime you have to buy with it - usually £10 - simply insert your new sim-only card and away you go (remembering to port your old number over).

Above are examples of the PAYG phones on offer at various prices.

Points to remember