Switching mobile provider? Here's one big mistake to avoid

Switching mobile provider can be one of the easiest money-saving moves you make, especially if you're coming off an expensive contract. 

But it's still important to know how the process works. Despite 79% of the 1,030 people who switched mobile networks in the past year agreeing the process was easy, there's one mistake in particular you'll want to avoid making. 

See where your provider ranks in our guide to the best and worst UK mobile networks.

What to expect when you switch mobile provider

Just saying 'it's easy' isn't enough to remove all doubt when it comes to potentially losing a mobile signal, so it's helpful to know in a bit more detail how a switch works.

First of all, don't worry – you can keep your number. To be able to switch provider you need what's called a PAC (porting authorisation code), and there's a specific code that tells the new provider to bring your old number with you. 

• To keep your number, you text PAC to 65075. 
• If for whatever reason you want a new number, you can text STAC to 75075 instead.

You'll then get a text (usually within one minute) with your PAC, which lasts for 30 days. Nothing's set in stone at this point – you're still not switching provider, you're just ready to do so, with your PAC code, within 30 days.

That text you receive will also tell you if you'll face any charges for early termination, which won’t apply if you’re out of contract.

Next comes the fun bit – finding out how much you can save by switching. If you haven't found a deal already, our mobile comparison service will let you easily compare, or you can browse our expert picks of the best Sim-only deals. 

Once you've picked a deal, your new provider will ask you for that PAC (or STAC), and contact your old network to set up the switch. Usually, this should happen the next working day.

So you can sit back and relax, but there's still that one big mistake you won't want to make until you're up and running. 

Use our mobile network coverage map to see which providers have the best signal in your area.

The mobile switching mistake people make

Making this mistake could leave you without connectivity for calls, texts or data while your number is ported. So here's what you should do to avoid it.

First of all, initiate the switch with your new provider on a Monday, Tuesday or Wednesday. That's because the switch should happen the next working day, so switching on a Friday, Saturday or Sunday could mean it takes until the following Tuesday for your switch to complete.

And importantly, don't change Sim cards until your old Sim stops working – this should mean everything has been transferred and your new Sim is ready to go. A common mistake is to swap it out immediately on the day of the switch, which could mean you lose signal and can't make calls, send texts or use data until it's completed. 

When you've switched Sim, check for those signal bars in your phone's taskbar and ideally a 4G or 5G icon as well (switch off wi-fi if you have it on to make sure). 

If anything's not working as it should, check the instructions you'll have received from your new provider carefully, as there may be some settings you need to change. Most of the time though, this should be done automatically. You can also restart your phone to help the new Sim bed in.

How much could you save by switching mobile network?

Switching may seem like a hassle but avoiding this key mistake should mean it goes smoothly. And don't forget the significant savings you can make by switching. 

Cheap deals with smaller providers such as Talkmobile, Giffgaff, Lebara and Smarty almost always beat the big four (EE, O2, Vodafone and Three). 

Our annual survey of thousands of mobile customers found that using one of the big four networks cost an average of £16 for a SIM-only contract, compared with just £9 on smaller networks. And for contracts that include a phone, the average cost was £40 with the big Four, versus £28 with smaller providers. 

See why our mobile providers expert says you shouldn't pay more than £8 a month. 

‘My only regret is leaving it so long’

We spoke to Jackie Gallego, Which? Home Editor, about how making 10 minutes of effort to switch networks saved her hundreds of pounds every year. Jackie said:

‘After a stern talking-to by Which? tech editor Paul Lester, I was finally convinced to ditch my O2 Sim-only mobile phone contract in November 2025. I’d signed up in 2018, lured by the promise of priority gig ticket booking through the O2 rewards app. It sounds like I was a truly loyal customer, but really, the painful process of switching back then left me unwilling to face it again in a hurry. Over the years, my bill slowly crept up from £17-ish a month to around £27, plus extras. One price bump was because I needed additional data, but the others were seemingly routine increases. 

'My new Sim-only deal with Smarty costs me just £8 a month for a chunky 50GB of data (an excellent Which? member offer), and it took me just 10 minutes to arrange. All I had to do was sign up on the Smarty website and request a porting authorisation code (PAC) from O2 so I could keep my old number. Number porting takes one working day during the week, so I made sure to get my request in well before the weekend. Smarty did all the rest. My Sim duly arrived in the post, I popped it in and I was good to go. The whole process was really easy and will save me at least £200 a year. My only regret is leaving it so long.’

For more tips on cutting your monthly costs, read our 10 ways to save money on your mobile phone bill.