Unfortunately, in backtrack you cannot use a dialup connection by DEFAULT...
But it is possible to use dial up connection using this tool: wvdial
so... you'll have to find a way of installing it.
After installation, shoot up a terminal & type wvdialconf.
This should show you the details about your modem...
Now goto file system >> etc
open the file wvdial.conf in text editor [You can find the file at the bottom of the folder]
now,.... enter phone number as *99# or as your ISP's number for dialing connection..