Personally, I would use a different function, a WHILE loop isn't going to perform well and nor is a scalar function. This uses Alan Burstein's NGrams8K: http://www.sqlservercentral.com/articles/Tally+Table/142316/.

After you have that function, you can create the function to remove the non-numeric values:
CREATE FUNCTION dbo.RemoveNonNumeric (@Phone varchar(8000)) RETURNS TABLE
AS RETURN

  SELECT (SELECT N.token + ''
    FROM dbo.NGrams8k(@Phone,1) N
    WHERE N.token LIKE '[0-9]'
    ORDER BY N.position
    FOR XML PATH('')) AS PhoneNum
GO


Now you can call it easily using APPLY. For example:
WITH VTE AS (
  SELECT *
  FROM (VALUES ('123-456,789'),
      ('1245,6957,4'),
      ('12346 647247')) V(Phone))
SELECT *
FROM VTE V
  CROSS APPLY RemoveNonNumeric(V.Phone) RNN;
GO