Sign 'Em Up: Stone Phillips is leaving ABC's "20/20" to co-anchor NBC's new prime-time news magazine with Jane Pauley, scheduled to debut in March. Jeff Diamond, former senior producer of "20/20," earlier was hired as the executive producer. . . . Country music star Garth Brooks will headline his first network special Jan. 17 on NBC. . . . Actor Sylvester Stallone, in a rare TV appearance, will host an "ABC in Concert" tribute to Elton John and Bernie Taupin Saturday from 9-10 p.m. . . . Producer Fred Silverman's company is planning a spinoff of "Matlock" to star comic-actor Max Alexander as a seemingly scatterbrained insurance investigator who disarms and outsmarts people.