Unfortunately at the present time there is no HCF modem driver for linux. Conexant have indicated in section 11 of their FAQ (http://www.conexant.com:80/cnxt/customer/md_faqs.htm) that they are currently developing one.
Several people have reported that they have got a HCF modem (in particular chipsets with the device ID of 1025) to work under Linux by using the HSF driver, however little firm information is known about this. If you successfully manage to use an HCF modem under linux please contact me to tell me about it.
One person has reported that if you get the "No Dialtone" error when trying to use a HSF driver with a HCF modem you can correct it by using the AT command "AT S7=45 S0=0 L1 V1 X4 &c1 E1 Q0".