Well, you could try something like:
^\d{9}(\d{2})?$
This matches exactly nine digits followed by an optional extra-two-digits (i.e., 9 or 11 digits).
Alternatively,
^(\d{9}|\d{11})$
may work as well.
But remember that not everything necessarily has to be done with regular expressions. It may be just as easy to check the string matches ^\d*$ and that the string length itself is either 9 or 11 (using something like strlen, for example).