When you arrive, the bank message sits in a SIM that you abandoned in a drawer in Birmingham. Your roaming ends, the SIM switches off and a one-time code for your credit card heads to a phone you'll never receive it on.
Why the SIM in your drawer stops working
Leave a UK pay-as-you-go SIM alone and it won't stick around. EE and Three become inactive after around 6 months of no chargeable activity, such as a paid call or a top-up (Vodafone is 180 days). EE offers three more months to "reactivate" by calling customer service, and then your number is lost forever.
This is the other trap: keeping it alive from outside the country. EU roam like at home rules have been in place since 15 June 2017, and were extended out to 2032 by the regulation currently in force as of 1 July 2022. This applies to the EEA, that is the 27 EU member states plus Iceland, Liechtenstein and Norway. The UK is not part of that, and fair use rules allow operators to charge extra for permanent roaming.
When the bank wants a code you cannot receive
The Strong Customer Authentication rules, which apply to online card payments in the UK since 14 March 2022, require a second step in most online payments and bank logins. For many UK banks, that involves sending a one-time passcode to your mobile phone by text message. The password is fine, the card is fine. But the text message arrives in a phone that is in a box in your parents' loft.
Same goes for jobs and for the state. Recruiters, GPs, banks, HMRC, almost anywhere you apply for a job or contact the government from the UK involves call and SMS from a UK number. Most websites would just reject a foreign number, requiring 11 digits and a leading 0. UK mobiles always begin 07, geographic numbers begin 01 or 02, and all 03 numbers cost the same as 01 and 02 to call.
How an app-based number works, and what it needs
No, a virtual phone number isn't a SIM. A SIM exists on your phone, this one sits on a server, and you receive it via app, so it rings on whatever phone you have with you. Select a country, select a number, and it's live. The problem is that the app needs an internet connection, be it data connection or Wi-Fi.
The good news is that a call to your number from anywhere does exactly the same thing. The UK number resting on a server and answering in Sydney works just like the one sitting in Sheffield, while the person ringing you from their house rings an ordinary 07 mobile number. Nothing on their bill is any different.
You can have both your home number and a local number. All iPhones from the iPhone XS, introduced in 2018, are eSIM-enabled, and the US models of the iPhone 14, introduced in 2022, have no SIM card slot. The default is to have one physical SIM and one eSIM active at the same time. It's more practical to have a local SIM for everyday use and the home number coming in through the app.
Where this quietly fails
Try entering a UK number into a GP surgery form and you might find it rejects anything that starts with +34. A UK number may receive a call or text from an HR department or HMRC, and web forms usually have a field that only accepts 07 and nine more digits. A human reader won't see it if it's on your CV, as the form rejects it without you knowing.
Most online card transactions and bank logins in the UK now require a second confirmation, Strong Customer Authentication (SCA). Numerous banks send that as a one-time-passcode by SMS to the mobile number associated with your account. If you buy virtual number for your bank and the app's switched off, you don't get the code. Test the critical ones first while your old SIM still works.
All you need is the number to verify. Once verified, WhatsApp works anywhere there's a data connection, even if the SIM for that number is absent. If you reinstall the app or move to a different device, you will need to verify again, that's a text at the old number. If you use a virtual number you'll discover that when you get a new iPhone overseas and the old SIM is stashed in a drawer.
What to check before you commit
Text PAC to 65075 before you leave the UK. Since 1 July 2019 this code gets sent within a minute and stays live for 30 days. This isn't a problem so long as the new network completes the switch within one working day of leaving; which is vital information if you intend to keep a network SIM. Check with your bank which number they use for passcodes, and ask if that number can be one running over an app.
If your phone supports e-SIM then use a UK SIM to receive bank codes, and get a second line. All iPhones since 2018 (XS or later) support e-SIM, while US iPhone 14 phones from 2022 have no SIM card tray. The majority of newer Android and Apple phones support multiple lines, with one SIM and one e-SIM. MoreMins supports phone numbers from over 50 countries in its app, with no SIM card. UK and US numbers from $0.99/month, no contract.
Questions people ask before moving
Will my bank accept an app-based number?
Because since 14 March 2022 your bank has to ask for a second identity check when you're about to buy something online, many follow up by sending a text. Call the bank in advance and see what it accepts.
Can I still use WhatsApp with a number I no longer have a SIM for?
Yep, until you do something that changes the situation. WhatsApp only requires a number when it verifies. After that it works anywhere with an internet connection, even without the SIM card, whether you reinstall the app or change phones (but then you have to verify again).
How long before a UK pay-as-you-go SIM dies?
About six months with no chargeable activity, but Vodafone will cut you off after 180 days and you lose any credit. EE offers three more months to get back online by calling.