Vodacom has allegedly been providing its subscribers' numbers to businesses and marketing companies, The Times reported on Thursday.
The cellular company reportedly confirmed on Wednesday that some of its 32.5 million local and 27 million international subscribers' personal details were supplied to other businesses.
The company launched an "urgent" internal investigation into the matter.
Vodacom spokesman Richard Boorman told The Times the company used "header enrichment" for select third party services, but that this was not its default operation.
Header enrichment is when personal data is transferred to a website when a user browses through a cellphone.
"We use it for a select number of Vodacom and trusted third-party services such as charge-to-bill," he was quoted as saying. Charge to bill is a built-in payment mechanism.
"We reassure our customers that their information is not being routinely shared with all websites."
Boorman reportedly declined to say whether Vodacom was being paid to supply the information, whether permission was obtained from subscribers, and who the third-parties were.
"We became aware of the issue late yesterday [Wednesday]. I will be in a better position to comment once the investigation has been completed," he said.