The best SIM-only deals in March 2023: offers on EE, Vodafone, Three and more (2023)

A SIM card has a couple of purposes. Most importantly, it is used to identify the phone contract you've signed up for, fulfilling your data, calls and texts plan.

A SIM plan also provides two numbers with overly complicated names - the ICCID serial number, International Mobile Subscriber Identity number (IMSI) and the Mobile Subscriber Integrated Services Digital Network number (MSISDN), the later of which is the user's telephone number.

In other words, a SIM card identifies your phone number and the SIM plan that you've subscribed to. Read our full SIM card storage guide to find out more.

As is usual for popular pieces of tech, SIM stands for something that sounds complicated but is quite simple. SIM stands for 'Subscriber Identity Module' - another way of saying a device to identify your phone deal.

SIM prices vary massively between networks and data caps and knowing how much to pay will depend on this. If you're not too worried about the network you're with and need next to no data, you can keep your bills to around £5-£8 a month.

However, going for EE SIM only deals - the UK's fastest and most popular network - will mean paying a fair bit more than the competition, often exceeding £20 a month. Decide how much data you require and consult your options based around that, considering both price and your preferred network.

While almost every single SIM plan will automatically offer unlimited texts and calls, data caps can range massively from 500Mb all the way through to unlimited data SIM only deals.

There are plenty of options in excess of 30GB, but the large majority of people are unlikely to need this much, usually using under 10GB each month. The easiest way to decide how much data you need is simply to consider how much you'd previously used on older contracts.

First time you're getting a SIM? These are how the data caps line up with usage:

• 1GB:A limited option that won't be enough for most. If you rarely use the internet and occasionally need to use Google Maps or check the internet, this can be a cheap option.
• 5GB-10GB:This is the range most people will fit into. It allows for pretty intensive usage of social media, Maps and internet searches without fear of running over. While you can do a bit of streaming, don't push it and avoid HD!
• 10GB-50GB:If you like to stream when you're out and about, use maps on a daily basis and generally have a data intensive lifestyle, this will likely be a cap you should be looking in. Keeping under 50GB can keep your costs at a still affordable point.
• 50-100GB:At this point, you're going to have to be a real data drainer for this to be necessary. Downloading apps on the go, HD streaming, big downloads and working from home.
• 100GB-unlimited:Unlimited data plans have significantly come down in price over the years and can even be a legitimate alternative solution to having full broadband if you live somewhere with an excellent 4G/5G connection. However, for most people, it's not going to be necessary.

There are four types of contract lengths available with SIM only deals - 1-month rolling, 12 months, 18 and 24. Realistically, 18-month contracts are pretty rare so the other three are the ones you'll see more.

24-month contracts tend to be the cheapest per month on big data plans but you're tied in for a while. 1-month rolling plans from the likes of GiffGaff, Voxi and Smarty are surprisingly affordable but you won't find them with the four main networks.

12-month contracts are the tariffs that come up most often and for the majority of people, will be the best choice.

A lot of SIM only deals will also come packed with cashback, giving you money back with your plan. This can significantly reduce your overall costs. This can come in the form of automatic cashback or cashback by redemption.

While cashback by redemption is the option that will save you more money, it is more complicated to deal with. Automatic cashback is a lot simpler, giving you all of your money back in one easy payment.

Gifts:
Slightly more rare, SIM plans can occasionally come with a free gift. These could be smart tech like speakers, TVs or tablets. We've seen everything from headphones through to smartwatches.

Of course, these aren't all that common and you need to be wary of a SIM plan that is offering a gift...but has included the price of the gift in your monthly bills.

SIM cards these days fall into three sizes and these are: Standard, Micro and Nano, descending in size. More on eSIMs below. If you're ordering your SIM only you'll likely find it arrives in a credit card sized piece of plastic out of which can you pop your microchip SIM.

A standard SIM measures 25 x 15mm and was used in phones before 2010 and a few after then.

The Micro SIM stands at 15 x 12 mm and was used in phones from 2010 and you'll still find some phones work with these now.

The Nano SIM, introduced in 2012, measures in at 12.3x 8.8mm and is the most commonly used SIM in phones.

The Combi SIM might be what you get sent by a SIM only provider as it contains all three sizes, and you push out the one that fits your phone. This is ideal as you don't need to know your size prior to this as you can hold it up to the SIM tray and see.

Switching phone plans has become a much easier process than it was in the past. Where before you had to go back and forth between your old and new provider, gathering details, now it's just a few steps.

Firstly, you need to know you're eligible to switch plans. If your contract still has a few months until it ends, you are likely to be charged for the remaining time meaning it is usually best to fully finish your contract first.

Switching and keeping your number:

Once you know when your contract is finishing and you've got your new deal, you need to decide if you want to keep your number. If so, you can request a PAC code (Porting Authorisation Code) from your old network. This can be done via the phone or through a text.

Don't want to keep your number - it's even easier! Simply get in contact with your new provider and tell them you want a new number and tell your old network your ending your contract.

This can also be done via a text to switch service. Simply text STAC to 75075.

If you're looking to get a new SIM plan for an old phone - whether its yours or one you bought secondhand - you could find that it is locked to a network. This is where a phone has had a SIM in it and now can only use that network.

While that sounds permanent, it's actually easily fixed. First, try putting a SIM card from another provider into your phone. Make a call and if you can't then its locked. You can also get in contact with your network to find out if its still locked.

How to unlock a smartphone:
Unlocking a smartphone is either a breeze or a bit more complicated depending on the network its locked to:

• EEOnce six months have gone by on your contract, you can call EE on 0800 956 6000 and pay them £8.99 to unlock your phone. It says it will take around 10 days to complete. PAYG phones can be unlocked for free. Both iPhones and Google devices unlock for free after a certain time with EE.
• O2Whether you have an iPhone or Android device, pay as you go or on a contract, O2 can unlock it for free. You simply need to log into the O2 app, select unlock my phone and you're good to go.
• ThreeFear not, all phones on Three are unlocked as standard. Shove whatever SIM you like in there, it will work a treat.
• VodafoneThe red network doesn't quite get the same marks as Three, as your phone will be locked to them on arrival. But they have made unlocking handsets absolutely free within 10 days of your request.