The way I can think of is running trixbox CE in a virtualbox. Do you have 1GB+ of RAM to run a virtual machine? You will need at minimum of 256MB of memory space for a VM to occupy in your RAM. That leaves 768MB of memory left. If you have 2GB of RAM in your laptop, you'll have have 1.7GB of memory space left once trixbox VM runs.
Unfortunately, a modem will not work with a virtual machine, as it's just like a separate PC, but it's virtualized in your laptop (VM stores in your hard drive). Not even a voice modem, as I assume your laptop only comes with data/fax modem.
A better idea is to build your own low-powered PC for purpose of running Asterisk.