Modems made with the PC-Tel 56k chipset are some of the cheapest currently on the market.  It is an HSP chipset which stands for Host Signal Processing.  In the .pdf file http://www.pctel.com/media/HSP_Whitepap.PDF, PC-Tel defines the HSP chipset as:

    "Put simply, HSP is a software-based technology that employs a portion of the unused instruction cycles  (MIPs) in a host computer's CPU chip's to perform signal processing and other functions that are normally handled by a separate Digital Signal Processing (DSP) or controller chip. This has been made practical by the rapid evolution of the microcomputer."

The lack of a DSP is a severe blow to the modem's functions.  They say that it only uses the unused instruction cycles, but these modems can actually use 50% or more of the CPU's processing power when dealing with low end Pentiums.  These modems are most commonly found in E-Machines.  Some manufacturers have updated drivers for these modems, but not many.  If the manufacturer of your modem does have new drivers and/or firmware it is HIGHLY recommended that you get them. 

    There is a basic procedure when trying to get a PC-Tel HSP modem to connect when having connection troubles or stopping it from getting dropped connections.  The first step is to download the latest drivers for your modem if there are any to download.  The second step is as follows:

Lower the port speed to 57,600, then lower the Fifo buffers by one setting on both transmit and recieve.  Then enter the initialization string:  AT&F&C1&D2&K3W1

If the modem is having further issues try dropping the Fifo buffers to the lowest setting and changing the string to:  AT&F&C1&D2&K3%N5W1 or AT&F&C1&D2&K3%N6W1

If these issues STILL persist change the init string to:  AT&FS37=12N1  The S37=12 sets the modem to v.34 mode and the N1 forces the modem to use the s-register number 37.  The final two inits that you can try are AT&FS37=12N1%N5 or AT&F&C1&D2&K3S37=12N1W1.

NEW!!!:  Try dealing with the s-register s91=#.  This register can actually compensate for your phone line's voltage level!  I would try s91=5 and then s91=15 and perhaps play around with it a bit.  The default value is s91=10.

   There are also several tricky things to try to get the modem to work.  If the computer has a S3 Video Card then try lowering the graphics resolution.  Also, if you are dialing into a cisco server, Cisco remote access has 2 available modems - Microcom and Mica. Recent Microcom firmware should be ok with the PCTel chipset, however Cisco MICA modems didn't support V.90 with PCTel until portware 2.6.2.0 which was released at the end of March 99. If a user is dialing into this type of site the portware needs to be upgraded or v34 must be used.  Cyrix processors will have a harder time with these modems than Intel processors.  There is a Cyrix firmware upgrade for the PC-Tel modems, but if you can't find updated drivers you probably won't be able to find this one.  There are no AMD drivers for the PC-Tel HSP's that I know of, the Intel drivers should hopefully work with these chips. 

   Additional notes and tricks to try:  Try adding commas(2-4) after the phone number.  Try ATZ.  Try changing W1 to W2.  Try %N4 instead of %N5 or %N6.  Try adding %E1 or %E3.  It is also possible that %E0 will help and changing flow control from RTS/CTS(hardware) to XON/XOFF(software) may actually help as well.  In fact, that makes sense in a way since it is a software modem.

Here is a partial list of manufacturer websites who make modems with the PC-Tel HSP chipset: