1. install ssh server on your linux box
2. then install minicom..http://alioth.debian.org/projects/minicom/ and configure it with the right serial port, speed, parity and etc. to interface with your modem

3. To access from a remote place, you can use putty as an ssh client, from thereon run minicom so you can access you modem. You can even use a mobile phone with wifi and java version of ssh client to access you serial modem...