There are basically two types of modems available, traditional hardware based modems and software based modems. Software modems have fewer chips on board, the tasks that would have been done by the missings chips are transferred to the host computer's main processor (the Pentium, etc..).
Two functions performed by any modem are:
US Robotics (now 3Com) manufactured Winmodems still have the datapump chip, but modem control functions are done by the host computer's main processor. This slightly adds to the load on the main processor. These modems are popular as OEM products, so when you buy a new PC, chances are that your modem is winmodem.
HSP modems (Host Signal Processing), transfer both datapump and modem control functions to the host computer's main processor. This greatly adds to the load on the main processor, which may be minimal on a 300MHz Pentium II, but, really slows down a Pentium 133.
In conclusion, software modems tend to be unreliable. A hardware modem offers more performance and reliability.