System and method for authorizing transactions, such as payments or money transfers. A source entity may initiate a transaction with a target entity via a first communications channel. In initiating the transaction, the source entity may indicate that the transaction is to be performed through a transaction authorization service. The target entity may send a transaction initiation message to the transaction authorization service. In response to receiving the transaction initiation message, the service may authorize the transaction with the source entity via a second communications channel. To authorize the transaction, the source entity may provide a PIN number or other identifier via the second communications channel. After receiving and validating the response from the source entity, the transaction authorization service may inform the target entity that the transaction is authorized. The target entity may then complete the transaction with the transaction authorization service and the source entity.