A couple of tricks here, from some recent work: a user-defined function to count the number of characters in a string, and a new (for me) syntax discovery: RETURN CASE.

Here's what I came up with for the user-defined function:

CREATE FUNCTION dbo.fnCountCharactersInString
(
    @StringToTest nvarchar(max),
    @CharactersToCount nvarchar(max)
)
RETURNS int
AS

BEGIN

    DECLARE @EditableCharacterCount int,
            @Looper int

    SELECT @EditableCharacterCount = 0,
           @Looper = 1

-- here's a readable version of this code for documentation purposes.
-- the code below is functionally identical, but performs faster
--   WHILE @Looper <= LEN(@StringToTest)
--       BEGIN
--           SELECT @EditableCharacterCount = @EditableCharacterCount +
--               CASE
--                   WHEN CHARINDEX (SUBSTRING (@StringToTest, @Looper, 1), @CharactersToCount) > 0
--                       THEN 1
--                   ELSE 0
--               END
--           SET @Looper = @Looper + 1
--   END

    WHILE @Looper <= LEN(@StringToTest)

        SELECT @EditableCharacterCount = @EditableCharacterCount +
               CASE
                   WHEN CHARINDEX (SUBSTRING (@StringToTest, @Looper, 1), @CharactersToCount) > 0
                       THEN 1
                   ELSE 0
               END,
               @Looper = @Looper + 1

    RETURN @EditableCharacterCount
END
GO

So, this query..

SELECT PhoneMask, dbo.fnCountCharactersInString (PhoneMask, '9#') FROM MyTable

.. might produce these results..

PhoneMask      (No column name)
(###)###-####  10
#-####-###     8
##-###-####    9
##-###-###9    9

In the production implementation of this work, I stumbled upon a nifty piece of syntax, RETURN CASE.  I needed to translate one potential value in my result set to another.  I tried this, and it worked:

RETURN
    CASE @EditableCharacterCount
        WHEN 0 THEN 64
        ELSE @EditableCharacterCount
    END

This works exactly as one would hope it would.  If the count of editable characters is 0, the value 64 is returned; otherwise, the true count is returned.  Another nifty implementation from the SQL team.