Accepted Values Stored in Variable
You need to declare @acceptedvalues as a table variable.  Similar to the below.

DECLARE @acceptedvalues AS TABLE 
(
AValue VARCHAR(50) NOT NULL
)

INSERT INTO @acceptedvalues (AValue) 
VALUES ('One'), ('Two'), ('Three')


Then it will work as you have specified in the case statement.

Sorry, you need to query the @AcceptedValues.

WHEN numinput NOT IN (SELECT AValue FROM @AcceptedValues) THEN 'Two' END AS [validnumber]