BT faces compensation claims from some of the UK's biggest companies following last week's network crash.
Customers of companies including NatWest Bank, the Virgin Group, the AA, British Gas, and Railtrack, were unable to get through on local call 0345 and 0845 numbers, and freephone number 0800 between 10.30am on Friday and 2.30pm on Saturday.
A NatWest spokeswoman told Computer Weekly: "Because customers couldn't get through for so long, we were inundated on Monday. We normally receive 250,000 calls a day, and had very much more than this on Monday onwards, as well as big queues forming.
"We have a meeting scheduled with BT, and compensation will be on the agenda because we have a service agreement." the spokeswoman added.
"We don't know how many NatWest customers may claim compensation from us, as a result of not being able to complete urgent payments over the phone."
Other companies confirmed they would seek compensation. NatWest said it was disappointed that BT took more than 24 hours to discover the problem, caused by a rogue switch. A BT spokesman admitted the company had no idea what the problem was until Saturday afternoon.
After isolating the problem, service returned to normal said BT, but the company is still waiting for the switch supplier to install new software to provide a permanent fix. The fault affected network gateways at Cambridge, Leeds, and Croydon, said BT, which refused to name the switch supplier.