A profoundly deaf woman who lost £8,371 when fraudsters took over her mobile phone and used it to empty her bank account, says her faith in the banking system – and Metro Bank in particular – has been left in tatters.

Louise Harte, who only uses sign language to communicate, says two months on she still has no idea how fraudsters were able to convince staff at the network Three to hand over her service, and how they then accessed her bank account.

The justice system worker is furious that bank staff flatly refused to discuss the theft with her and her son – who is a sign language interpreter – when they went in to a branch.

Her case will shock mobile banking users who receive codes to check if payments are valid. It also exposes how some banks refuse to refund unwitting victims of fraud – and the obstacles suffered by deaf or disabled people when dealing with big companies.

Problems began for Harte, from Edgware in London, in July when she returned from holiday in Cyprus and discovered her mobile phone no longer worked. She now knows that while she had been abroad, someone had gone into the Three mobile store in Woolwich, south-east London, posing as her. They somehow persuaded staff to give them a new sim card with her number. The reason her phone no longer worked was because her service had been swapped to the new sim card – that was in the fraudster’s possession.

Within a few hours, the same gang logged into her Metro account from a computer in the Maidenhead area, and removed the £8,371. To authorise this transfer, Metro sent a verification code to Harte’s mobile, but as this was in the hands of the fraudster, it went through. However, a second payment was blocked “as suspicious”.

Harte and her son say they have been blocked at every turn. When she initially visited the Three store on London’s Oxford Street, staff failed to disclose that the sim-swap had even taken place. She says she was left “humiliated” by staff who forced her to talk over the phone in front of other customers.

“I have never felt so small and embarrassed. I can only liken it to forcing a wheelchair user to walk when forcing a deaf person to speak,” she told Money.

Staff at the bank also refused to talk to her son, despite Harte asking staff to add him to her account a year previously.

“I am deaf but the bank did not take this into consideration and made no attempt to adapt to my needs,” she says. “This led to confusion, misunderstandings and, frankly, atrocious customer service.”

After four weeks of worry and disputes, the bank revealed (at a meeting when it brought in an interpreter) that it had been able to recover £5,700 of her money, but the rest, she was told, was lost.

Metro refused to refund her losses, arguing that only she had access to her online bank details, with the implication that she must have been negligent. This echoes an earlier case Money reported in February, when the bank refused to refund a Kent businessman who lost £20,000 when his EE mobile service was taken over by fraudsters.

Last week, the Financial Ombudsman Service warned it would no longer tolerate banks refusing refunds in cases where the consumer had no role in making the payment out of their account.

The good news is that following Guardian Money’s intervention Metro has now refunded Harte the £2,671. It has also offered her £250 compensation, but she has dismissed this as derisory.

“I still have no idea how or why fraudsters have targeted me,” she says. “I don’t understand how someone was able to walk out of a Three shop with my mobile service, or to log in to my bank account. I simply don’t trust these firms to look after my interests.”

A Metro Bank spokeswoman said: “We understand and appreciate the impact fraud has on victims and we have taken the opportunity to fully review Ms Harte’s case. Our investigation has shown that a fraudster was able to ascertain her online banking security credentials and her card details. Although we made every effort to take her disability into account, by communicating with her over email and offering to arrange meetings with a sign language interpreter present, we are sorry that we didn’t meet her expectations during her time of need. As a gesture of goodwill, we have therefore decided to refund the outstanding £2,671.”

A Three spokesperson apologised and admitted her experience had not been up to its “usual standards”. It has offered few details and no compensation. “We have been looking at ways to improve our service for deaf customers, having recently partnered with Sign Solutions to pilot an in-store video relay service which we hope to roll out more widely in future.”