I use dial up and the used motherboard I built my system with didn't have a COM/Serial port, so in order to use my serial modem, I had to buy a USB to serial converter cable. Since using this modem set up in the past year, my internet speeds haven't been too great, and the past two months or so have been frustrating. Frustrating enough to even give a couple of other ISP's a try - still no improvement.
My Best Data modem died, and my USRobotics modem isn't the easiest to configure So, I searched for another modem and heard good things about the Multi-Tech MultiModemZBA-USB series. I tried the newest one they have, the 9234 model, thinking it was LINUX compatible. It really isn't, unless you want to recompile your kernel and modules. I returned it and traded it in for a refurbished 5634ZBA-USB v.92 unit and it works right out of the box using the ACM0 module. The baud speed is much faster than either my Best Data or USRobtics. The only setting I fiddled with in KPPP is the pre-init and post-init delay times. I extended them from 50 to 68 and now I connect without problems and I am amazed at the surfing speed-very impressive for dial up. So, if you use dial up. look for one of these units no longer being made. I'm going to email Multi-Tech and implore them to bring this modem model back as it works great in LINUX. Why they stopped making it, I'll never understand. I have found a few linux information bug related pages and the problems the few developers are having getting this newer unit to work with LINUX.
I bought my refurbished unit from modemexpress.