That looked right to me at first, but on second thought doesn't it just provide the last two digits? That is, if A1 has "6677889900", your formula will return not "6677889905" (which is what I suppose the OP wants) but just "05".
Pretty close, though; easy enough to fix it:
Code:
=LEFT(A1,8)&SUBSTITUTE(RIGHT(A1,2),"00","05")