I use KPPP in KDE. I did have to change a setting in the file "/etc/ppp/options". I commented out the line for "proxyarp".
Before I did that I was getting errors about proxy arp failing to determine an IP address for the network interface. I don't know if that was causing a problem because I had other problems at the time. It was confusing the issue by causing an error message.
If you don't want to totally disable proxy ARP you can include the option in the extra ppp options for the dial-up connection in KPPP.
Try a few different phone numbers for your dial-up provider before you conclude that dial-up isn't working. Mine had some numbers that worked better than others. You may also want to check to see that the two modems are actually negotiating a connection. Listen on the phone line or enable the modem speaker. You should hear a lot of hissing and pinging noises. Eventually that should stop (after a minute or less). If it keeps going until the line disconnects then there's something incompatible with the modem settings. I had to disable V.90 on my modem to get it to work with my ISP over my home phone line. Listening to the phone line with a regular phone may interfere with the modem communication or cause the line to drop so try at least a few times without picking up another phone extension.
I should point out that the big problem most people have with Linux and dial-up is the software modems used in computers. They are often modems like Connexant that do everything in a Windows driver. Most of them don't work with Linux though a few can with a lot of effort.
You can use the terminal in KPPP's modem settings or query the modem to tell if it is responding to commands through the serial port. Finding the correct modem device can be difficult. It is often "/dev/modem" but it can also be other things like "/dev/ttyS0", "/dev/ttyUSB0" or "/dev/ttyACM0". If your computer has one or two regular COM ports then the number may be 1 or 2 instead of 0.
If Linux does not detect your modem and you've tried all the software solutions, here is a good product that I use with Linux. It's easy to install and works great. It requires no special drivers.
U.S. Robotics USR5637 56K USB Modem
It's a good investment since it works with any laptop or computer that you want and is quite compact. Since you can only use one dial-up link there's not much point in putting a modem in every computer.
For desktop computers the other hardware U.S. Robotics cards have always worked well for me with Linux. They are more expensive than the U.S. Robotics (and other) software modems. You can find other brands but be careful that you're actually buying a real hardware modem and not one with only partial hardware support for modem functions. You usually have to spend around $50 or more to get a hardware modem.
There is a web site called "Linuxant" with some drivers for software modems. The free versions have limited features but are enough to tell if the drivers will work for you. The registration for the full versions is only around $20 or so and might be worth it if the drivers work really well for you. I finally decided to just get a hardware modem. The Linuxant drivers didn't work with newer versions of my Linux distro and also required many kernel settings that disabled newer kernel features. I had none of those problems using a hardware modem.