xDSL n Dial up connection ??? help

[MoonblueZ]

          ----------         -----------
          |  ISP 1 |     |  ISP 2  |
          ----------         -----------
              |                    |
  (DHCP) |                    | (DHCP)
              |                    |
             -----               -----
Dial up -->  |   |               |   |  <-- xDSL
             -----               -----
                \               /
                ----------------
                |   router     |
                ----------------
                       |
                       |
               --------------------

I have some problem here, about how to setting freebsd router which have two
link like topology above. i want the router have automatically detected if
xDSL link has down n swicth the link to dial up automatically. even so was
the reverse if link xDSL has up n dial up is disconnect automatically..

thnx