Telstra has admitted that like Optus, it hands over customers' mobile phone numbers to certain partner websites visited by those customers on their mobile device without their knowledge or consent.
As first reported by ZDNet on Wednesday, Optus confirmed that it hands over customer numbers to some websites it has a commercial relationship with, such as premium service providers, via an unencrypted HTTP header of the web request to that site, through a practice known as HTTP Header Enrichment.
"When consumers browse the internet, information about the device they're using is passed on to website owners in order to optimise websites for those users," the spokesperson said at the time.
"Optus adds our customers' mobile number to the information in select circumstances where we have a commercial relationship with owners of particular websites."
The spokesperson defended the practice, stating that it is common in the industry.
At the time of reporting, ZDNet had sought information from Telstra and Vodafone as to whether they too pass on customer phone numbers.
On Thursday, Telstra confirmed that it is also engaged in the practice.
"We only provide our customers' mobile numbers to a small number of our content partners to enable the customer to choose to receive the service and pay for it via the Telstra bill. We provide this information on the basis of legal terms in our partner contracts that help ensure the privacy of our customers," a spokesperson for Telstra told ZDNet.
Vodafone confirmed on Thursday that it does not hand over numbers to partner sites automatically, but only once a user agrees to terms and conditions for having a purchase added to their phone bill. A spokesperson for the company insisted that the data is encrypted when passed onto the partner site.
"We take the safeguarding of customer information very seriously, as it is one of our biggest responsibilities," the spokesperson said.
"Carrier billing is a convenient and secure billing option offered by mobile carriers worldwide."
ZDNet understands that Optus has been approached by the Office of the Australian Privacy Commissioner about the practice.
UK mobile provider O2 was discovered in 2012 to be passing mobile numbers not only onto "trusted partners" including premium content providers, but also inadvertently to a number of other sites.