A method comprising: a) determining, by a computer, a current geographic location of a mobile communication device of a user, the mobile communication device being associated with an account number for a payment account; b) forming, by the computer, a boundary function using the current geographic location of the mobile communication device, wherein the boundary function serves to separate a geographic area from another area; c) determining, by the computer, a plurality of pre-defined geographic zones contacting or within the boundary function, the plurality of pre-defined geographic zones respectively associated with a plurality of pre-defined geographic zone identifiers; d) receiving, by the computer, transaction data for a current transaction and determining a pre-defined geographic zone identifier for a pre-defined geographic zone from the transaction data, wherein the transaction data is present in an authorization request message generated and transmitted by an access device in a transaction comprising an interaction between a portable user device of the user and the access device, the authorization request message comprising payment credentials, or a payment token associated with payment credentials, that are associated with the payment account associated with the mobile communication device; e) determining, by the computer, that the pre-defined geographic zone matches one of the pre-defined geographic zone identifiers in the plurality of pre-defined geographic zones identifiers; and f) in response to step e), either i) providing a match indicator to an authorizing entity computer indicating that the mobile communication device is proximate to the access device or ii) authorizing the transaction to proceed. a) determining, by a computer, a current geographic location of a mobile communication device of a user, the mobile communication device being associated with an account number for a payment account; b) forming, by the computer, a boundary function using the current geographic location of the mobile communication device, wherein the boundary function serves to separate a geographic area from another area; c) determining, by the computer, a plurality of pre-defined geographic zones contacting or within the boundary function, the plurality of pre-defined geographic zones respectively associated with a plurality of pre-defined geographic zone identifiers; d) receiving, by the computer, transaction data for a current transaction and determining a pre-defined geographic zone identifier for a pre-defined geographic zone from the transaction data, wherein the transaction data is present in an authorization request message generated and transmitted by an access device in a transaction comprising an interaction between a portable user device of the user and the access device, the authorization request message comprising payment credentials, or a payment token associated with payment credentials, that are associated with the payment account associated with the mobile communication device; e) determining, by the computer, that the pre-defined geographic zone matches one of the pre-defined geographic zone identifiers in the plurality of pre-defined geographic zones identifiers; and f) in response to step e), either i) providing a match indicator to an authorizing entity computer indicating that the mobile communication device is proximate to the access device or ii) authorizing the transaction to proceed.