The numbers, which have now been taken down, were listed following the introduction of the BT Together unmetered phone service, which allows customers to make unlimited calls from home for a flat fee.
BT is trying to prevent the package being used for free Internet access. It publicly listed numbers that received heavy use, in the belief that they were ISP numbers and they would be charges at standard rates.
However, remote workers who used the BT Together service and some of the numbers listed, were ex-directory and gave access to corporate networks.
With hackers sometimes finding it harder to uncover corporate access numbers than to crack weak passwords, the security implications are clear.
A BT spokesman said the network log-in numbers were now no longer being published on the BT site, and that users could now input their numbers into the site to check whether they were free to use or not.