There are three kinds of modems available: internal, PCMCIA card or external serial port modems. But some internal modems will not work with Linux these modems are usually called WinModem. This is caused by non-standard hardware. So you have to use either a PCMCIA card modem or an external modem (serial or USB). The LinModem-HOWTO by Sean Walbran provides a detailed instruction how to deal with these kind of modems. My pages about Internal Modems in Laptops and miniPCI Devices in Laptops provide a survey about the modem controllers used in different laptops.
Quotation from the Kernel-FAQ: "9.Why aren't WinModems supported? (REG, quoting Edward S. Marshall) The problem is the lack of specifications for this hardware. Most companies producing so-called WinModems refuse to provide specifications which would allow non-Microsoft operating systems to use them. The basic issue is that they don't work like a traditional modem; they don't have a DSP, and make the CPU do all the work. Hence, you can't talk to them like a traditional modem, and you -need- to run the modem driver as a realtime task, or you'll have serious data loss issues under any kind of load. They're simply a poor design."
"Win modems are lobotomized modems which expect Windows to do some of their thinking for them. If you do not have Windows, you do not have a connection. "
Anyway, I have set up a page collecting information on laptops with internal modems at TuxMobil - Hardware . Maybe it's possible to run such modems with MS-Windows9x/NT emulators like wine or VMware, but I don't know it.
The Linux WinModem Support and the Xmodem.org (former Gromit Winmodem) page are more or less the standard as to whether a modem is real or not, and also contain directions to getting drivers for the few winmodems that do have Linux drivers.
Pay attention to the different kinds of phone lines: analog and ISDN. You can't connect an analog modem to an ISDN port and vice versa. Though there might be hybrid modems available. Connecting to the wrong port may even destroy your modem. Trick: If you are looking for an analog phone port in an office building which is usually wired with ISDN, take a look at the fax lines, they are often analog lines.
If your machine features an internal modem as well as an internal ethernet card, pay also attention to plug the right cable into the plug. Otherwise you may damage your hardware easily. It may even cause a fire.
For tracking the packets on PPP you may use pppstats. Or pload this provides a graphical view of the traffic (in and out) of the PPP connection. It is based on athena widgets hence is very portable. It also uses very little CPU time.