A method is provided for completing an authenticated commercial transaction over an internet protocol (IP) network (40) for an account holder (60) engaged in the transaction via a non-IP based telecommunications platform (30). The method includes: receiving a first message from the account holder (60) via the non-IP based telecommunications platform (30), the first message triggering authentication of the account holder (60) and being in a first communication format; establishing account information for a payment instrument being used in the transaction based upon content in the first message; generating a second message using a second communication format different from the first format, the second message including the established account information; submitting the second message via the IP network (40) to a network entity (110, 200) such that an authentication document is generated (120), the authentication document (120) containing an input field (122); generating a third message using the first format, the third message being submitted to the account holder (60) via the non-IP based telecommunications platform (30) such that the account holder (60) is prompted to enter a security code; receiving a fourth message containing the entered security code from the account holder (60) via the non-IP based telecommunications platform (30), the fourth message being in the first format; and, accessing the authentication document (120) via the IP network (40) to fill-in the input field (122) of the authentication document (120) with the security code contained in the fourth message.