When you get a new SIM card, either from a shop or in the post, you'll need to activate it before it can be used in a phone. Most often the process is fast and simple, but it does vary according to the supplier.
First things first, though, you'll need to make sure your phone isn't locked to a different network. If you bought your smartphone on contract from Vodafone, for example, it is probably locked and will only work with Vodafone SIM cards. You might be able to get you phone unlocked for free - see our guide to unlocking an iPhone (it also applies to other makes).
If you're travelling to the UK from abroad the same applies: you'll need an unlocked phone to use a UK SIM card.
There are two main sizes of SIM card: micro and nano. Full-size SIM cards are still used by some phones, but they're rare these days. From the iPhone 5 onwards, Apple switched to nano SIM cards, but most Android phones use micro SIMs, including the Sony Xperia Z, HTC One M8 and Samsung Galaxy S4.
You can use a nano SIM in a phone that takes a micro SIM by using an inexpensive adaptor, but a micro SIM is too large to fit into a nano SIM slot.
Here are the three SIM card sizes so you can check which you have:
Using an adaptor is slightly risky depending on the way the SIM is inserted into the phone. Generally they're fine when there's a removable tray, but can be very hard to remove when it's a manual push-in mechanism. Never insert an adaptor without the SIM card in the centre as it will get caught on the metal prongs inside the slot and could do irreparable damage to the phone when you forcefully remove it.
How to activate a SIM card:
It's best to turn your phone completely off before removing the current SIM and inserting the new one. Some phones - notably iPhones - let you 'hot-swap' SIMs but you might find others need a reboot before they will recognise you've removed and replaced one.
Activating a SIM isn't usually necessary with EE as they are pre-activated if you have just joined EE, moved from Orange or T-Mobile or you were sent a replacement SIM due to theft, loss or damage to your old SIM.
You can activate your SIM online if you're a pay monthly customer, but for pay-as-you-go SIMs it's different. If you got the SIM in a Vodafone shop, it will be activated already. If you ordered it online, call 17298 after inserting it into your phone.
If you have a Pay Monthly O2 SIM, it comes pre-activated so you merely have to insert it.
For Pay & Go SIMs, you'll need to first top up http://www.o2.co.uk/help/pay-and-go/topping-up
The O2 Top-Up number is 4444 or +441753 552 277
You'll need to create a Giffgaff account if you don't have one already. Head to the activation page and enter the six-digit code on the card containing your SIM. Most SIMs are activated within seconds, but it can take up to 24 hours in extreme circumstances.
Insert the SIM and turn on your phone. Dial 5588 and you should hear a voice telling you the SIM is now active.