Cellnet introduced its 'Callback' service on its mobile phones in 1991. The aim of the campaign by GGT Advertising was to launch the new service to the public and to encourage customers to subscribe to Cellnet. The structure of the mobile market did not allow network providers to sell directly to the customers but they had to go through service providers, were thus meant to remain anonymous commodity suppliers. The Callback campaign introduced discrimination in as duopoly market and thus created a brand. The advertising campaign directly resulted in increased profits. By 1992 12-15% of Cellnet users used Callback. Airtime usage was estimated to have gone up by 10%. Further value-added services were planned