What some O2 customers may have wanted for Christmas was a phone number, since long-cherished ones have had a habit of going astray. Whether they got lucky is another matter, as O2’s customer services can be as hard to pin down as Lord Lucan.

JD’s number was taken from her when fraudsters, pretending be her, tricked O2 into transferring it to a sim they’d acquired from another provider.

This enabled them to receive texts, including two-factor authentication codes sent by banks to verify that a customer is who they say they are. As a result, more than £4,500 was promptly stolen from her credit card. O2 explained she had been a victim of “sim-swap fraud”, where criminals transfer your phone number to their sim to receive your calls and texts – including those from banks.

It promised to block her stolen number and dispatch a new card. This arrived and JD’s phone sprang back into life – but only for a few hours. Unbelievably, fraudsters managed, once again, to divert the number to their own sim. O2 admitted it had failed to flag the first sim-swap as fraud, which meant the second was waved through unquestioned.

JD says she was repeatedly told that the fraud team would contact her, but they did not. When, five days later, she managed to get hold of them, they had closed the case.

Compensation was not due, she was told, because O2 had done nothing wrong. By then, she had been without a working phone for nearly two weeks. Her bank refunded the stolen £4,500, but she was unable to access any of her bank accounts because she could not receive the required security codes to log in.

O2 claims it was a coincidence the number was restored the day I made contact, but acknowledged its communications had been poor and has offered £350 in goodwill.

• When AW’s 91-year-old mother lost the pay-as-you-go (PAYG) number she relies on, O2 told her that her sim did not match the number in question. AW rang back. “I was told to wait 48 working hours as a form needed to be filled out,” she writes. “Six days later, I was told there was potentially an account takeover and it would take 10 working days to fill out two more forms.

“Ten working days after that, I was transferred to the retention department but, again, told that, because the number and sim did not match, they could not do anything. I was put through to the fraud department and reached a recorded message before being cut off.”

O2 decided the only way the number could be restored would be for the account to be switched from PAYG to a monthly contract. This was done, a new number was provided, and AW cancelled the contract within the cooling-off period.

Whereupon O2 leapt into action. Not to resolve the complaint, but to demand and chase an unexplained £9.35 fee. Of the £24 credit stranded in the old PAYG account, there was no word. O2 told me it had made multiple attempts to reach AW’s mother to resolve the issue. I asked when and how the company had realised it had been ringing her on the long-lost number.

It then blamed the delay on the fact the PAYG account had not been fully set up. How come, I asked, since it had been functioning flawlessly for years. What O2 apparently meant was that customers setting up a monthly contract have to provide full details – which, since the account in question was not a monthly contract, is irrelevant.

Belatedly, it decided that AW’s mother might have fallen victim to sim-swap fraud, but it could not be sure, as it does not hold enough information about PAYG customers.

It has now refunded the credit, paid £110 in goodwill for the poor service and offered, five months late, to restore the lost number – but AW’s mother has now, unsurprisingly, switched providers.

• AF alerted O2 after receiving an unrequested PAC (porting authorisation code). This allows you to keep your old number with you when you change service provider. O2 told him it needed 10 working days to investigate. His phone number then stopped working and he was told it had been ported. Over two weeks later he called O2 three times and visited a branch – to be told, on the third attempt, that he had not picked up a call, so the case had been closed.

O2 tells me he was to blame as it had tried three times to contact him just before closing the case. He too was a victim of sim-swap fraud, it has confirmed, and his number has since been returned, but he won’t be compensated because, as O2 predictably declares, it was all his fault.

Telecoms regulator Ofcom requires verification of a customer’s identity before a number is ported. It says it has been monitoring complaints about sim-swap fraud involving O2, which is ranked bottom of its latest performance table due to poor complaints handling.

It says: “We’ve discussed steps they are taking to protect customers,” adding: “It’s vital that all communications providers protect their customers, and if we see evidence of widespread harm, we’ve shown we won’t hesitate to act where appropriate.”

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