Hi @DJChoice,
There's only two ways that your number could be used in this way. The first would be through a SIM swap fraud, i.e the fraudster had your number swapped to an alternative SIM. This can't have happened if you're still getting the return calls, as your number can only be active on one SIM at any one time.
So the most likely explanation will be something called "Spoofing". You can read more about spoofing on our Fraud to watch out for page. Spoofing is done with software which hides the real number being called from, and presents a number of the fraudster's choosing instead. Most numbers spoofed are picked at random, and are perpetrated by fraudsters outside the UK, they'll have most likely chosen your number simply to appear as though they're calling from the UK.
Organisations like Action Fraud and OfCom are working on solutions to tackle this situation, but it's a complex issue which is difficult to solve. Furthermore the fraudsters don't actually need any access to your number to fake calling from it.
We appreciate this can be really frustrating. You can have your phone number changed by customer services if you get in touch, however in most cases spoofing doesn't go on for long, as the perpetrators tend not to use the same number for many calls.
I'm sorry we can't do much to help with this other than provide information, hopefully we'll be able to work with the relevant authorities to put a stop to this in time. If you have any concerns that your account or number may have been accessed by a 3rd party, please get in touch with customer services, and they can double check this to be certain, but as I mentioned, spoofing is done independently of your network.
Thanks,
Jonathan
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