BT has been fined £225,000 for a lack of transparency over the cost of calls to its 118500 directory enquiry line.
BT was also reprimanded for dragging out calls unnecessarily to increase the charges faced by people calling the helpline.
The ruling came after a number of consumers reported 118500 to the watchdog after suffering 'bill shock'.
A spokesperson for PhonepayPlus said the regulator requires all premium rate services to make sure the price is "clearly displayed" in their adverts and that callers are not "unduly delayed" on the phone call.
"PhonepayPlus takes action if any company fails to meet the requirements of our Code of Practice," the representative stated.
BT issued an apology to affected customers, saying that "oversights and different interpretations of the code" were to blame.