HSBC customers are being forced to wait up to an hour and a half on hold every time they try to contact the company.
For weeks, all callers to the bank's main customer service number have been presented with a recorded message.
"We are experiencing high call volumes and currently customers are waiting in excess of 25 minutes for their call to be answered," it says. "You may prefer to call back later".
Frustrated customers are venting their anger on the bank's Facebook page and many are threatening to leave.
"This is totally unacceptable and ridiculous. All I want is a contactless debit card. Simple. Get a grip HSBC or I'll be off to a bank that does answer calls from its 'valued' customers," says Mick Vardy.
"You can pay $10bn to shareholders and £7m to [HSBC chief executive] Mr Gulliver, try paying for a few new phone lines and people to answer them.....quickly."
At least one customer has been stranded abroad without access to cash and unable to get through to the bank. Others face massive mobile bills.
Many say they've been forced to wait for over an hour and a half before the phone is answered, and one says she was on the phone from nine thirty in the morning until quarter past two.
HSBC is refusing to give any explanation, or any idea of when things might return to normal.
"We are aware that call waiting times have crept up, and we are sorry that customers have been affected by this," a spokesman says. "We are taking this extremely seriously and are actively revising our service to ensure more calls are answered in a timely fashion."
However, the extreme delays have been going on for over a month now, with the situation getting no better. As customer Holly Oliver points out, "What I find interesting is that no one seem to care about this - nothing is done to improve the situation! Does your CEO read this page? Do the shareholders?"
People can't necessarily bypass the delays even if they visit their branch instead, as counter staff are often forced to phone the same 0345 number for help - and suffer the same long wait.
And while customers are begging HSBC to take on more staff to cut waiting times, the company is doing just the opposite. It last year unveiled a restructuring plan that will see one in five jobs lost around the world. Eight thousand British jobs will go by the end of next year.
Meanwhile, in February, chief executive Stuart Gulliver pocketed more than £6 million in bonuses on top of his £1.25 million salary.