An online advertisement, offering a monthly contract for an iPhone XR for £36, has been criticised by the Advertising Standards Authority.
It follows complaints from some who placed orders but were told they could not be processed.
Carphone Warehouse said that unprecedented demand created a backlog with some people failing credit checks in error and having orders cancelled.
The ASA said users should have been notified about the issues.
The ad, part of a Black Friday promotion, appeared on mobiles.co.uk - a division of Carphone Warehouse.
"If promoters relied on being able to meet the estimated response but were unable to supply demand for a promotional offer because of an unexpectedly high response or some other anticipated factor outside their control, they must ensure relevant timely communication with applicants and consumers and, in cases of any likely detriment, offer a refund or a reasonable substitute product," it said in its ruling.
It added that the online ad should not appear again in its current form.
The original promotion offered the latest Apple handset with 100GB of 4G mobile data for £36 a month and no upfront cost.
A spokesperson for mobiles.co.uk said: "We're disappointed by the ASA's decision but we take on board their recommendation to increase the level of communication to customers if something similar were to happen again."
Mobiles.co.uk said that it had worked with network provider Vodafone to resolve the issue.
The problem seemed to rest with Vodafone's credit check system which was incorrectly turning down some applications. The BBC understands about 5% of customers experienced an issue.
Customers had also complained that calls to the helpline cost 13p a minute, with some being on the line for more than an hour without the matter being resolved.
Mobiles.co.uk said it had provided a press statement and an email address for customers who had been affected, which resulted in many previously declined transactions converting to a sale.
The ASA contended that the statement was released after the promotional period, and that it had no evidence that customers had been notified during the period itself.
It added that customers were not made aware there was a likelihood that the order could not be processed, and customers were not told to contact the website to have their credit checks rerun.