It is not universally known, but almost always you can save money in the long run by buying a smartphone SIM free, and then choosing a data- and calls package on the basis of the cost of the SIM alone. So if you are buying a new phone go for the SIM free option. But if you already have a phone you like, are out of contract, and want to choose any SIM package, you need to unlock your phone. Here we explain how to unlock your phone so that it can take any SIM. (See also: The 30 best smartphones: The best phone you can buy in 2014.)
1. How to use any SIM in your phone: Buy an unlocked phone
The easiest way to use any SIM in your handset, is to buy a smartphone that is unlocked and SIM free. This may sound like an obvious point, but it is only easy when you know how. Phones are typically sold in three distinct ways: on contract, pay as you go, and SIM free. And with a SIM free phone you can choose your network on the basis of price of data and calls, although it is likely the phone itself will cost a bit more. Of course, you are reading this because you already have a phone into which you want to put your choice of SIM.
2. How to use any SIM in your phone: Unlock your smartphone using a code
There are two ways of unlocking an existing smartphone. The hardware method using a cable is always an option, and we'll outline it below. But much easier is to unlock your phone via input of a software code. First you need to get together the required information, and then find or purchase that code.
The information you need is your phone's brand and model number, the network to which it is locked, and the IMEI code. We'll presume you know the phone model and the network. To find the IMEI *#06# into your handset as if you were making a call. You won't have to hit 'call', just note down the unique (and long) number that appears.
Presuming that your phone is relatively modern, the first call should be to your current network. Although the network won't want to lose your custom, they should offer to sell you an unlock code. If your phone can be unlocked your network will give you a quote for a code. It is also possible that your phone can't be unlocked, but your network can at least tell you that, and save you trying to find a code elsewhere. The cost of an unlock code can vary depending on handset and network, but if your phone can be unlocked expect to pay around £20.
According to Moneysavingexpert Vodafone give for free unlock codes to customers who have been on contract for more than a year, and charges £20 to all other customers. O2 also lets contract customers unlock handsets for free, regardless of their tenure of custome. With O2 it is £15 if you've been a PAYG customer for over a year. O2 iPhone customers can pay £15 to unlock at any point.
If your network can't sell you a code - or charges too much - but you can unlock your phone via this method, you could shop around for a cheaper code. You know those 'Phone unlock' shops on the high street and stalls on the local market? This is what they are selling. Some shoe repair- and key cutting stores also offer this service. You can also find such codes online but this is a murky world into which to dip your toe. Personally we'd rather buy from a store to which we can return if the code doesn't work, but if you are looking online, be sure you know exactly what you're getting before you pay.
Finally, if you have an older phone, you may find that you can re-use a code from an older handset. And the good news is that communally minded phone users sometimes post these online for free sharing. Sites such as GiffGaff's Unlockapedia are worth a visit, then. The chances are that you won't be able to find a free code, but you may as well have a go.
Once you have a code you now need to use it to unlock the handset. This process is pretty simple. Just pop in a SIM card for any network other than the one to which your phone is locked. You'll be prompted to put in your code. You may be in luck and be able to use your normal qwerty keyboard, but more likely you'll be able to input only numbers in this way and the letters will require you to use multiple presses of the * and # keys. (You will get only five attempts to enter the correct code, but getting it wrong won't brick your phone. It will just mean that you have to take it back to the network to get it unlocked).
Once you are successful you will get a message telling you that the phone is unlocked. Find a cheap SIM and away you go! (See also: How to dry out an iPhone.)
3. How to use any SIM in your phone: Unlock your smartphone using a cable
If you can't unlock your smartphone using a code, you may be able to do so using a cable attachment. You can either pay to have it unlocked this way - we're back on the high street or at the local market. Again, we'd rather see the whites of the eyes of the person we are paying to mess about with our smartphone, so we'd tend to go local.