How does a SIM only work?

SIM only deals give you a phone number and mobile access, just without the handset. They're often cheaper than phone contracts and can offer more flexibility.

Some people prefer to have a mobile phone contract where the handset is included. In this guide, we’ll cover how SIM only deals work, the pros and cons, and whether it’s worth switching.

What is a SIM?

A SIM (short for Subscriber Identity Module) is the small card that lets your phone connect to a mobile network.

You can also get an eSIM, which works the same way but is embedded in your phone, no plastic card required.

Your SIM holds essential info, including:

• Your phone number
• Network and user ID
• Contacts and text messages
• Location and call data

SIMs are tied to network providers like O2, EE, Vodafone and Three, and can’t be swapped between networks.

What is a SIM only deal?

A SIM only deal gives you mobile minutes, texts and data, but not a new handset.

You pick the plan, the network sends the SIM, and you pop it into your existing phone. Just check your phone’s unlocked, especially if you bought it before December 2021 (when networks stopped selling locked phones as standard).

There are two main types of SIM only plans:

• Pay monthly - A rolling 30-day contract, or sometimes 12–24 months. You get a set monthly allowance of calls, texts and data. If you’re committing long-term, make sure it’s worth it
• Pay as you go (PAYG) - Top up when you need to. No contract, no credit checks. You only pay for what you use

Should I get a SIM only deal?

If you're thinking about switching to a SIM only deal, here's a quick breakdown of the pros and cons:

Advantages	Disadvantages
Cheaper overall – SIM only deals cost less because you're not paying for a phone. The Guardian reported in April 2025 that you could save up to £350 a year if you're happy to keep your current handset.	High upfront cost for new phones – Buying a handset outright can be expensive, often hundreds (or even thousands) of pounds.
Shorter contracts – Many SIM only deals run on 30-day rolling contracts, so you can switch providers easily if needed.	Phone may be locked – Older handsets (bought before December 2021) might be locked to a network. You’ll need to unlock it before switching. Most providers will do this for free.
Keep your current phone – Happy with your handset? Just insert your new SIM. No need to upgrade unless you want to.
More flexibility – Upgrade your phone when it suits you – you're not stuck waiting for the end of a contract.
Easier credit checks – SIM only contracts are generally easier to get. Some pay-as-you-go SIMs don’t need a credit check at all.

Planning to switch? Here’s how to keep the same number when switching networks.

How do I get my SIM to work?

Getting your SIM working is usually as simple as popping it into your phone.

SIM cards come in three sizes. Just snap out the one that fits your handset:

• Standard SIM: 15 x 25mm
• Micro SIM: 12 x 15mm
• Nano SIM: 8.8 x 12.3mm

Or, you can go for an eSIM, a virtual SIM that doesn’t need to slot into your phone. Find out how eSIMs work in our dedicated guide.

Is SIM only cheaper than pay as you go?

Usually, yes. But it depends on how much you use your phone.

With SIM only, you pay a monthly fee for a set bundle of minutes, texts and data. Pay-as-you go (PAYG) charges you per call, message, and megabyte, and you pre-load your account in advance.

SIM only plans tend to be cheaper per unit, but it’s worth doing the maths to see what works best for you.

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

Switching to SIM only before your contract ends usually means paying an early termination fee. This fee depends on how many months are left and can add up.

Weigh the fee against how much you’d save on SIM only to decide if it’s worth switching now or waiting.

Is it worth going SIM only?

If you can get the same data, texts and minutes for less, and your current phone is working fine, SIM only is probably a smart move.

But it’s not for everyone. Make sure you weigh the pros and cons before deciding.

Getting the right SIM for you

Travelling abroad? Check out the best SIM only deals for roaming.

Like to stream on the go? Take a look at our SIM only deals for unlimited data

Looking for a bargain? Here’s how to cut your mobile bill with cheap SIM only deals .

Looking to cut your spending? Here’s how to lower your mobile bill.

Compare SIM Only Deals and find the one that suits you best.