Using disposable phone numbers for better security

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Last night I received a call from my bank. They'd detected an unusual transaction and wanted to make sure that it was legitimate. Had I recently purchased £10,000 worth of crypto in the Maldives? What?!!? No! ARGH!

I started to panic. All my apes money gone!

No. Wait. The other thing. I knew it was a scam from the moment "James from your bank's fraud team" started his patter.

You see, I have multiple phone numbers. And "James" called me on a number which isn't tied to my bank. So I strung him along for half an hour or so pretending to move my money into a safe account, taking ages to wait for an SMS confirmation code, accidentally reading out the wrong one, before cursing him and his lineage even unto the seventh generation. He, in return, introduced me to a whole new range of swearwords from his native tongue and hung up on me. Charming!

Here's how it works, and how you can use this trick.

The easiest way is to get a dual SIM phone. I have my main SIM which holds my primary phone number. That's the number used by my family, my banks, and anyone vaguely trustworthy. My second SIM contains a free disposable PAYG SIM.

That phone number gets given out to retailers, couriers, Gumtree & Freecycle users, pizza delivery, and anyone else who doesn't need my number permanently.

My Android phone tells me which line is being called. And once I start getting too much spam to that number I throw the SIM away and get a new one with a new number.

If you have an eSIM, you can do the same thing. Find a cheap eSIM provider, sign up to a PAYG or monthly plan, then ditch the number whenever you like.

Or, you can set up a SIP calling plan. Install an app and have calls automatically routed to you.

About a dozen years ago, I worked with a UK mobile network to develop "disposable" phone numbers. The idea was that we would partner with, say, a dating app and generate a new phone number for you. Your date could call and text you without you having to reveal your real number. If they turned out to be a jerk, you could revoke the phone number immediately.

The same tech could work for hiring a plumber, getting a takeaway, or a hundred different use cases. The plan was to have an app which displayed a push-notification telling you which number was being called - so you knew if it was from your temporary lover or the person picking up your old sofa.

Sadly, the demo never went anywhere. It's a pity. I'd love to have a SIM with multiple disposable numbers.

But, for now, give out your temporary number to people who don't need a way to permanently contact you. Better safe than sorry!

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7 thoughts on “Using disposable phone numbers for better security

  1. @Edent I once set this up too - with landline numbers on my mobile.

    Had fun demonstrating - I could text an area code and have a new number assigned on the spot, and then make or receive calls using it.

    Managing this as a service without it being massively abused it not so simple, sadly.

    I did consider offering to the local lost dogs group (when we lost a dog), but fraudsters would claim to have lost a dog just to get a disposable number.

    Sad we live in such a world some times.

  2. @Edent I would happily pay for a SAAS subscription that was an app that generated mobile numbers and then routed them back to ring the app on my phone

  3. James (not from the fraud team) says:

    About the idea of providing this as a service, while reducing negative externalities: I wonder whether a dedicated -- and tightly controlled -- UK area code for 'ephemeral phone numbers' would help make a service like that less appealing to fraudsters, yet socially acceptable for situations like food deliveries, first dates, and so on.

    It wouldn't allow fobbing someone off with the expectation that you'd given them a permanent phone number.. but perhaps that's a feature?

    It'd also have to be a well-publicised and easy-to-remember number range -- and there are probably second-and-third order effects I'm not considering, but it springs to mind as a potential way to deal with some of the potential problems.

  4. a says:

    Can't two sim phones mute or turn off the second sim? So you won't receive anything on it until you unmute it.

    1. @edent says:

      Yes, they can. But I don't always know when a courier is going to call me with an update on my delivery. So I tend to keep it on.


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