What happens to old phone numbers? | Nathaniel Read

What happens to old phone numbers?

An interesting question of logistics, is that of phone numbers and how they’re handled in the UK, how’re they managed and what problems does increased connectivity bring?


In the UK SIM cards are not per region, all mobile numbers are in the form 07XXX XXXXXX, whereas in North America they are per-area, in the UK whether you’re in England, Wales, Scotland or Northern Island your phone number will be of the same form.

When you get a new SIM card from a new phone company, either pay as you go or pay monthly, you’ll be allocated a new phone number or you’ll have the option to bring your old number if you have one. But if you choose a new number, what happens to the old one?

The Number Shortage

Smart Meters, soon to be commonplace in the UK require a SIM card to connect over the Internet (a big problem with the Internet of Things (but that’s been discussed in a previous post), meaning that 27 million [ONS] additional phone numbers will be needed just for Electricity and a further 27 million for Gas, meaning that 54 million additional phone numbers would be needed for these devices to connect to the internet. As a result, Ofcom are considering adding additional digits for these devices numbers to potentially free billions of additional numbers.

The shortage of phone numbers is added on to by the hoarding of inactive numbers by providers, where only 15.4% of O2’s 152.3 million numbers active and EE with only 20.6% of 133.6 million SIMs active. (Source)

Phone Number Deletion

The actual management of numbers isn’t managed by Ofcom (the UK’s phone regulator, networks are only told to use numbers ‘effectively and efficiently’. As a result, how long until your phone number is deleted depends on your service provider, but your number being deleted only happens if you haven’t made a call, text or accessed the internet on your device within the following amounts of time:

This point are when your number is deleted, your credit is usually deleted and SIM card cancelled before this point. Source
EE O2 Three Vodafone
180 days 6 months 6 months 6 months

Number Reallocation

There are no real rules on reallocation of numbers either, the only law is that they are quarantined for 6 months, but variation from this can be seen in that my own old Vodafone number from 5 years ago is still disconnected and not reallocated.\

When the phone number stops being yours however, it waits in quarantine and then returns to the reallocation pool where it will either be returned to an inactivate SIM (as can be seen by tills in supermarkets) or a new person’s contract.

There are problems with reassigning numbers if people have had them previously even if they have been left disused for a year. Repeated nuisance calls or texts from people looking for the previous owner will only ring once but spam callers or even debt collectors could try to contact for years to come.

Fringe Cases

Phone numbers are provided in blocks of 100,000 to providers when they start to run out of numbers. You can be see the blocks each provider owns here or you can reverse lookup your own here. But if you have phone number with EE and then port it to O2, the number returns back to the original provider (O2) to be reallocated.


Ofcom expects providers to responsibly manage their numbers responsibly and reallocate their existing numbers before asking for new ones. The rise of the Internet of Things devices through devices such as smart meters or even 4G equipped CCTV cameras are going to require a rethink of the number allocation system, and potentially giving these devices longer numbers or a different type of identifier.

  Figures and data accurate as of August 2017
  Phone icon adapted from Apple's iOS