Depending upon your source, minimalism ended in the mid-1970s. Tell that to Yoko Ono. The notorious Beatle widow has never been one to heed the dictates of the art world, as she demonstrated in a recent landmark retrospective at the Museum of Contemporary Art. The piece? A simple white Slimline telephone, with an equally simple instruction posted nearby: "When the telephone rings, pick up the receiver and talk to Yoko Ono." And for the show's three-month run, Ono made random calls and chatted with whoever answered. You can't get much more minimal than that.