Removing alphabetic characters in SQL Server

Below is the function below

create function NoLetters(@string2number varchar(200)) 
 returns varchar(200)
as
begin
  declare @c int
  declare @num varchar(200)
  set @num = ''
  declare @txtCurrent char(1)
  set @c=1
  while @c<=len(@string2number)
    begin
     set @txtCurrent = substring(@string2number,@c,1)
     if ascii(@txtCurrent) between 48 and 57
       set @num = @num + @txtCurrent
     set @c=@c+1
   end
 return @num
end