Ofcom today fined Barclaycard £50,000 for breaching rules on silent and abandoned calls, the regulator announced.
The fine, the maximum Ofcom is allowed to impose, follows an investigation between October 2006 and May 2007 which found Barclaycard made an "extremely high number of silent calls".
The investigation also found that some of Barclaycard's call centres had no procedures to prevent people receiving repeated abandoned calls over a short period of time.
Ofcom chief executive Ed Richards said: "Taken as a whole this is the most serious case of persistent misuse by making silent and abandoned calls that Ofcom has ever investigated.
"Had we not been limited by the statutory maximum, we would have imposed a larger financial penalty to reflect this misuse."
Ofcom said those who received the silent calls from Barclaycard had no way of knowing who had made them.
Silent calls were a significant cause of inconvenience and anxiety for thousands of people every month, the regulator said.
Most were not generated with malicious intent but were made when call centres using automated systems placed more calls than their agents could deal with.
Ofcom regulations say abandoned calls must make up no more than 3% of all live calls in a 24-hour period and all abandoned calls must carry a short recorded message identifying who it is from.
All calls from automated calling systems must also allow people to dial 1471 to find the telephone number of the person or organisation calling them.
Ofcom has previously fined Abbey National, Complete Credit Management, Space Kitchens, Bracken Bay Kitchens, Carphone Warehouse and Toucan for breaching rules on silent and abandoned calls.
Barclaycard apologised to its customers today and said it accepted the ruling.
It said in a statement: "Barclaycard does not dispute Ofcom's findings and accepts the resulting fine.
"Many of these calls were made with the intention of bringing potentially fraudulent activity to the attention of the card holder.
"Nevertheless, we recognise that all calls, irrespective of the purpose, should be made in the right way and we accept that our processes, in place at the time of the review by Ofcom, were inadequate.
"As a result, we offer a full apology for any inconvenience and distress to our customers that these calls caused.
"Barclaycard is operating within Ofcom's rules. We have made robust and lasting changes to our processes, operations and reporting to ensure that we continue to be compliant and to provide the highest levels of service to all our customers."