System and method for authorizing transactions, such as payments or money transfers. A transaction authorization mechanism may be provided through which a transaction initiated via a first communications channel may be authorized through a second, separate communications channel or mechanism. A source entity may initiate a transaction to a target entity via a first communications channel to a transaction authorization service. The transaction authorization service may confirm the transaction with the source entity via a second communications channel. Various embodiments may use different communications channels, for example conventional telephone systems, mobile/cellular phone systems, and text messaging systems as the first or second communications channels. After the transaction has been authorized via the second communications channel, the transaction authorization service and/or the source entity may contact the target entity to inform the target entity of the transaction. The target entity may then complete the transaction.