It depends on the type of MODEM. Please post the output of the command lspci which will let us know the type of chipset. Since analog MODEMs are all winmodem class these days not all are supported.
Some background information:
http://linmodems.org/
http://www.faqs.org/docs/Linux-HOWTO...nux-HOWTO.html
If your MODEM is not supported then consider purchasing a USB ZOOM model 3095. I do not know how well it works but it is supported. You might be able to find a used external MODEM which requires a serial port or USB-serial adapter.
There could be other options depending on your location. There could be Pixius or similar service. It is basically wireless. An antenna is mounted on the roof which must be line of sight of the main tower. The output of the receiver is ethernet which connects to a regular router.