Replace Vanity Phone Numbers with Numbers

create function replacePhoneNumberVanityChars(@string varchar(max))
returns varchar(max)
begin

 declare @len as int = len(@string);
 declare @i as int = 1;
 declare @char as varchar(1) = null;

 while(@i<=@len)
 begin

  set @char = right(left(@string,@i),1);

  if(@char in ('A','B','C'))
   set @char = '2';
  if(@char in ('D','E','F'))
   set @char = '3';
  if(@char in ('G','H','I'))
   set @char = '4';
  if(@char in ('J','K','L'))
   set @char = '5';
  if(@char in ('M','N','O'))
   set @char = '6';
  if(@char in ('P','Q','R','S'))
   set @char = '7';
  if(@char in ('T','U','V'))
   set @char = '8'; 
  if(@char in ('W','X','Y','Z'))
   set @char = '9';
  
  set @string = left(@string,@i-1) + @char  + right(@string,len(@string)-@i);

  set @char = null;
  set @i = @i +1;

 end
 
 return @string;

end