Didn't Majel "only" do alternate episodes of Voyager? Who did the other episodes?
I think you're probably thinking of Enterprise
. On Voyager
, Majel always did the Federation computer. On Enterprise
, she only voiced the computer in the mirror universe episodes. But that's not because they cast someone else, it's because the computers on the NX-01 didn't speak. And also, of course, on DS9 they had Judi Durand do the Cardassian computer on the station, but Majel still did all the Federation computer voices, like the Defiant and the runabouts.