How does SIM only work? Everything you need to know about SIM only deals

Stack of SIM cards

SIM only deals give you a phone number but no phone. Here we look at how SIM only deals work, their pros and cons, and whether it’s worth going SIM only.

While some opt for mobile contracts which include a handset, going for a SIM only deal works better for many phone users. The best SIM only deals are usually cheaper than handset contracts, and afford more flexibility.

Here we look into how SIM only deals work for mobile phones and why you might choose one. Plus we answer the all-important question: ‘should I switch to a SIM-only mobile phone deal to save money?’

What is a SIM?

As you’re no doubt aware, a SIM (which stands for subscriber identity module) card is what slots into your phone or device. It’s a smart card that stores certain information and authorises your phone to connect to a mobile network.

It’s also possible to get an eSIM, which does the same job except it’s virtual, and there’s no piece of plastic to insert into your phone.

It has important information, such as your:

  • User identity
  • Phone number
  • Location
  • Contacts
  • Call history
  • Stored text messages

This is supplied by a network provider, such as Three, O2, EE or Vodafone. While your handset may not be, the SIM is specific to a network and can’t be switched over.

What is a SIM only deal?

If you want to connect to a mobile network but don’t need a new handset, a SIM only deal is the way to go. 

Once you’ve decided on the SIM deal you want, you’ll be sent the card and then you can use this in the handset of your choice. Bear in mind that some handsets are locked to certain networks, particularly if they were first bought before December 2021. But otherwise you should be good to go.

In terms of how SIM only deals work, you tend to get two types:

  • Pay monthly. Here you sign up for a rolling contract, often a month or 30 days. This gives you a monthly allowance of calls (commonly known as minutes), texts and mobile data for browsing and apps.
    Some SIM only deals may come with a longer contract, such as 12 or even 24 months. If you’re happy being tied in for a long period, make sure it’s a sweet deal before you commit.
  • Pay as you go (PAYG). This is where you only pay for your usage. Just top up your phone with credit – such as online or in person at Post Office branches – as and when you need it. You’re not tied into a contract and often these don’t require a credit check.

Should I get a SIM only deal?

If you’re considering getting a SIM only deal, here are the pros and cons.

Advantages of a SIM only deal

  • SIM only deals are cheaper. This is the main benefit of going SIM only. As other contracts need to cover the cost of a new smartphone, they’re invariably more expensive. But with SIM only, the main costs are those of the minutes, texts and data usage. In fact, figures from comparison website Uswitch found that switching to a SIM-only contract can save you approximately £335 a year.
  • You’re not tied to one provider for a long period. As many of the contracts are as short as 30 days, you can switch providers very easily if they don’t meet your expectations.
  • You can stick with your smartphone. If you have a handset that you’re particularly fond of but want to switch to a better package, a SIM only deal will allow you to just pop a SIM card into your existing phone. It may be locked to a certain network, but most networks let you unlock your phone for free.
  • Upgrade to a new phone when you like. On the flipside, you don’t have to wait until you’re due an upgrade before getting a shiny new phone. Just pop the SIM into your new device and you’re good to go.
  • Easier to pass the credit check. SIM only credit checks are a lower bar than contracts featuring handsets, as people are less likely to default on their payment. And with some PAYG SIMs, there are no credit checks at all.

Please note that if you do plan on hopping between SIM only deals, here’s how to keep the same number when switching networks.

Disadvantages of a SIM only deal

  • New smartphones are expensive. If you’re buying a handset outright, it can be eye-watering. Most new smartphones cost somewhere in the hundreds and can even get into the thousands. So one of the main advantages of a contract which comes with a handset is that you spread the cost.
  • Your phone may be locked. As mentioned, handsets are occasionally locked into a specific network, although this has been banned since December 2021. But if the phone is older than that, it means a SIM from a different provider may not work. If this is the case, you’ll need to get your phone unlocked. Most major networks do this for free though.

How do I get my SIM to work?

With SIM only deals, getting the SIM card to work is normally just a matter of popping it into your phone.

Your SIM is likely to come in three different sizes. You just pop out the right size for your handset. These are:

  • Standard SIM: 15 x 25mm
  • Micro SIM: 12 x 15mm
  • Nano SIM: 8.8 x 12.3mm
Three SIM card sizes. Illustration of a Mini-SIM, a Micro-SIM, and a Nano-SIM.

It’s now also possible to get an eSIM, which is a virtual SIM that you don’t need to slot into your phone. Find out how an eSIM works in our article.

Is SIM only cheaper than pay as you go?

This depends on how often you use your phone. But in most cases, it’s likely to be. 

With a SIM only deal, you pay a monthly fee and you’re given a set amount of minutes, text and data to use. PAYG deals, on the other hand, charge for calls, messages and data on a per-text, per-minute and per-megabyte basis. You’ll need to pre-load your mobile account in advance, and charges are deducted accordingly.

SIM only deals typically work out cheaper on a per unit level, although this may vary on a case-by-case basis. Always do your sums to work out what’s the best value for you before committing.

I’m still in a contract. Can I end it early to go SIM only?

If you switch to a SIM only deal before your existing contract has come to an end, you’ll pay an early termination fee. This will be based on the remaining months in the minimum term of your contract. It could work out to be a significant sum.

Weigh up the size of the fee against the amount you could save by switching to a SIM only deal. This way you’ll see whether it’s worth paying or if you’re better off waiting until your contract ends.

Is it worth going SIM only?

If you’ve done the maths and worked out that you can get an equivalent bundle of data, texts and minutes for a cheaper price – and your current mobile handset is working well – it’ll likely make sense to go SIM only. 

However, it won’t be right for everyone. You need to weigh up the pros and cons of SIM only deals before deciding which option is best for you.

Getting the right SIM for you

If you’re planning on travelling abroad, check out the best SIM only deals for roaming. Here are the best SIM only deals for unlimited data and here are the best SIM only deals if you just wanna go cheap.

Looking to cut your spending? Here’s how to lower your mobile bill in seven easy steps.