I have successfully configured the Toshiba 1605CDS modem and tested under Redhat Linux 6.2 using KPPP.

Here are the instructions on how I did it... 

I tried several different methods before I got it working so if this doesn't work just email me (mavic@mediaone.net) with details and I can help you out further.

1.  Make sure you are logged in or su to get root privileges.

2. Download hsflinmodem-4.06.06.01mbsibeta01121400.tar.gz

3. Unpack the hsflinmodem-4.06.06.01mbsibeta01121400.tar.gz 

    -  #tar zxvf hsflinmodem-4.06.06.01mbsibeta01121400.tar.gz

    -  #cd hsflinmodem-4.06.06.01mbsibeta01121400

4.  #make install

5.  #hsfconfig

    -  use the defaults of this config program.

6. USE

    - #kppp to start kppp

    - the modem should work with any ppp program but I have use kppp so I will use this.

    - your modem should now be /dev/modem

    - hit the "Setup" button.

    - configure your account (dial number username/password etc...)

    - under the device tab make sure Modem Device is set to /dev/modem, leave the other options as they are unless you know what your doing.

    - under the modem tab hit query modem to make sure it is working. Hit ok then connect. It should work !