After 18 months at the head of AT&T Data Systems, Vittorio Cassoni is returning to Olivetti to be group managing director, AT&T announced yesterday. AT&T senior vice-president and chief financial officer Robert Kavner will succeed Cassoni. Kavner joined AT&T from accountants Coopers & Lybrand in 1984, where he had been responsible for client relations. The loss of Cassoni, who was effectively on secondment from Olivetti, will be keenly felt because he had just begun to give the computer unit a sense of purpose. Following IBM’s example, AT&T is to try to persuade 1,500 administrative staff to forsake their warm offices for the rigours of the road. The 1,500 invited to apply to transfer to sales are in Network Operations and Business Markets.