Altere o
RANK para ROW_NUMBER . SELECT *
FROM ( SELECT ID, Phone, [LastDate],
ROW_NUMBER() OVER (PARTITION BY Phone ORDER BY [LastDate]) AS 'RANK',
COUNT(Phone) OVER (PARTITION BY Phone) AS 'MAXCOUNT'
FROM MyTable
WHERE Groupid = 5) a
WHERE [RANK] = [MAXCOUNT]
