55 Electronic messages and traffic data may only be processed by a natural person employed by or acting on behalf of a communications provider or subscriber for the purpose of processing electronic messages and traffic data to perform the functions referred to separately in this Chapter. Section 138 Processing data for the purposes of transmitting communications and implementing a service and ensuring information security Messages and traffic data may only be processed to the extent necessary for transmitting communications, implementing the agreed service and for the purpose of ensuring information security as provided in section 272. Communications providers and added value service providers shall inform subscribers or users about what traffic data are being processed for the service referred to in subsection 1 and how long the processing will last. Section 139 Processing for billing purposes Communications service providers may process traffic data necessary for defining fees between themselves and for billing purposes. An information society service provider may process traffic data received from a telecommunications operator which is necessary for the billing of image recordings, sound recordings and other fee-based services offered over a communications network administered by that telecommunications operator, and any other data necessary for billing. Information society service providers are entitled to obtain this data from telecommunications operators. Billing-related data must be stored for a minimum of three months from the due date of the bill or the saving of the traffic data, whichever is later. Such data must not, however, be stored beyond the time the debt becomes statute-barred under the Act on Statute-Barred Debt (728/2003). However, in the case of a dispute over a bill, the data pertaining to that bill must be stored until the matter has been settled or resolved and entered into legal force. Communications providers shall inform subscribers or users about what traffic data are being processed and how long the processing will last. Section 140 Processing for marketing purposes A communications provider may, for the purpose of marketing its services, process traffic data to such an extent and for such a period of time as the marketing requires if the subscriber or user to whom the data applies has given consent thereto. The party giving such consent must have the opportunity to cancel his or her consent regarding the processing of traffic data. Section 141 Processing for the purposes of technical development