For modems, a good idea is to check the Chipset on the modem. There is pretty good support for Conexant chipsets and I have had a lot of luck getting them to work. To locate your chipset, you may want to see what 'lspci' brings to the table. If that doesn't give you anything useful, pull out the modem and check the chip on the modem, note: you may have to pull the goofy sticker that some modem manufacturers place over the controller chip.