Most people pick a mobile deal once and barely look at it again. The trouble is, the option that feels cheap each month often costs you the most over time. So before you sign anything, it's worth doing the actual maths.
How the Two Options Actually Work
A SIM-only deal gives you minutes, texts and data, nothing else. You put the SIM in a phone you already own, or one you've bought separately. A phone contract bundles a handset and an airtime plan into one monthly payment, usually over 24 or 36 months.
The key difference is what you're paying for. With a contract, part of your monthly bill is quietly paying off the cost of the phone. With SIM-only, you're paying for the service and nothing more. That's why SIM-only plans are almost always cheaper month to month.
Running the Numbers on a Contract
Here's where it gets interesting. Take a current iPhone that sells outright for around £800. A typical 24-month bundled contract on a big network can come in at well over £900 once you add the upfront fee, so you're paying a noticeable premium for the convenience of spreading the cost. Buying the handset outright and pairing it with a cheap SIM-only plan usually works out cheaper across the same two years, and the gap widens if you go with a smaller network.
The bigger problem is what happens when the contract ends. Most people don't switch. They keep paying the full monthly amount even after the phone is paid off, which means they're handing over a large sum purely for the airtime.
If you want a low monthly cost without that trap, a basic SIM-only deal will do the job. For example, a simple £5 SIM-only plan will usually give you 5GB of data, plenty of UK minutes and texts, no contract, and no credit check needed.
That kind of plan suits more people than you'd think. If you spend most of your day on Wi-Fi at home or work, you barely touch your mobile data. Ofcom's latest Communications Market Report puts the average UK mobile data use at 10.6GB a month, and plenty of people use far less than that.
How the Big Networks Pull You In
The UK now has three main mobile network operators, EE, VodafoneThree (formed when Vodafone and Three merged in June 2025) and Virgin Media O2. They tend to nudge you towards a new handset and a longer commitment.
EE, for example, lets you swap to a new phone before your contract ends through its Upgrade Anytime scheme, but early upgrades in the first year come with fees of up to £600. The others run similar early upgrade offers. It sounds generous, but it often means starting a fresh contract before you've finished paying off the phone you already have.
Then there are the mid-contract price rises. Under Ofcom's new rules from January 2025, these have to be set in pounds and pence, not as a percentage. EE, for any SIM-only or airtime plan taken out from 31 July 2025, now adds £2.50 a month every March, and £4 a month for handset bundles. VodafoneThree and O2 apply their own annual increases on similar terms. Many smaller networks, including Lebara, SMARTY and VOXI, currently run deals with no in-contract price rises at all.
It's worth knowing that smaller networks, called MVNOs, rent space on the big three's masts. SMARTY uses Three's network, giffgaff uses O2, and Lebara runs on Vodafone. You're often getting the same coverage for less.
Who Each Option Really Suits
This all comes down to how you use your phone and what you can afford upfront. A contract makes sense if you want the latest handset and can't put down hundreds of pounds in one go, or if you're trying to build a credit history. A few things to weigh up:
- Buy outright plus SIM-only if you can afford the handset and want the lowest long-term cost.
- A cheap SIM-only plan if you already own a phone and use mostly Wi-Fi.
- A contract if you need a new phone now and prefer spreading the cost.
There's no single right answer. Someone who streams video on the train all day needs a very different plan from someone who mainly texts and checks emails.
Do the Maths Before You Sign
The honest answer is to add up the total cost over the full length of any deal, not just the monthly figure. A £30 contract over 24 months is £720 before you've added the phone or any price rises. A £5 SIM in the same phone is £120.
Work out what you genuinely use, check whether you're still paying off a handset you already own, and pick the option that fits. The maths nearly always rewards the people who bother to do it.
