Barclaycard has been hit with the maximum possible fine - £50,000 - by UK regulators for making an "extremely high number" of silent and abandoned calls to cardholders.
Silent calling occurs when contact centres using automated phone systems dial more numbers than staff can handle, leaving the line quiet when the recipient answers.
The watchdog says silent calls "are a significant cause of inconvenience and anxiety for thousands of people every month".
In rules published in 2006 Ofcom says abandoned call rates must be no more than three per cent of all live calls made in a 24 hour period. Abandoned calls must also carry a short recorded message identifying the source of the call, whilst recipients should be able to find the number of the person or firm contacting them by dialling 1471.
But, following an investigation between 1 October 2006 and 10 May 2007, Ofcom found Barclaycard made an "extremely high number" of silent calls where the recipient was unable to check who made them.
In addition, the company had no procedures in place to prevent people receiving repeated abandoned calls over a short period of time.
Ed Richards, chief executive, Ofcom, says: "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 has previously fined Abbey National, Complete Credit Management and Carphone Warehouse for breaching the silent calling rules.