Siemens AG, which has about 50% of the West German market for cellular telephones, and is working with L M Ericsson on the new generation digital cellular equipment for the new system due to come in across Europe from 1991, has formed a joint venture with France’s $700m-a-year Societe Anonyme de Telecommunications, SAT, to develop digital mobile phones. SAT’s Radiotel unit manufactures cellular phones in France.