I think something roughly like this:

(SELECT COUNT(DISTINCT daysworked) AS 'Days Worked'

FROM (SELECT CAST(DATEPART(MM, DATEADD(HOUR, -8, ActualEnd)) AS VARCHAR) + '/' + CAST(DATEPART(DD, DATEADD(HOUR, -8, ActualEnd)) AS VARCHAR) + '/' + CAST(DATEPART(YYYY, DATEADD(HOUR, -8,ActualEnd))

AS varchar) AS daysworked, ActivityId AS totalcalls

FROM PhoneCall AS p

WHERE ActualEnd >= DATEADD(HOUR, -8, DATEADD(QUARTER, DATEDIFF(QUARTER, 0, GETDATE()) - 1, 0)) AND

ActualEnd < DATEADD(HOUR, -8, DATEADD(QUARTER, DATEDIFF(QUARTER, 0, GETDATE()), 0)) AND

(OwnerId = x.SystemUserId)) AS tb)

AS [Days Worked],