MYSQL - Consulta de tabla plana

Necesitas definir un GROUP BY y use la cláusula MAX agregado para obtener el resultado que desea:

  SELECT listup.NodeNumber As Node, listup.Station As Extension,
         MAX(CASE WHEN VoiceServer = 1 THEN KeyDescription ELSE NULL END) AS 'Key1',
         MAX(CASE WHEN VoiceServer = 2 THEN KeyDescription ELSE NULL END) AS 'Key2',
         MAX(CASE WHEN VoiceServer = 3 THEN KeyDescription ELSE NULL END) AS 'Key3',
         MAX(CASE WHEN VoiceServer = 4 THEN KeyDescription ELSE NULL END) AS 'Key4',
         MAX(CASE WHEN VoiceServer = 5 THEN KeyDescription ELSE NULL END) AS 'Key5',
         MAX(CASE WHEN VoiceServer = 6 THEN KeyDescription ELSE NULL END) AS 'Key6',
         MAX(CASE WHEN VoiceServer = 7 THEN KeyDescription ELSE NULL END) AS 'Key7',
         MAX(CASE WHEN VoiceServer = 8 THEN KeyDescription ELSE NULL END) AS 'Key8',
         MAX(CASE WHEN VoiceServer = 9 THEN KeyDescription ELSE NULL END) AS 'Key9',
         MAX(CASE WHEN VoiceServer = 10 THEN KeyDescription ELSE NULL END) AS 'Key10'
    FROM listup
GROUP BY listup.NodeNumber As Node, listup.Station As Extension
ORDER BY listup.NodeNumber, listup.Station

NULL es preferible a una cadena de longitud cero.