The following information was provided by a customer, with an older USR Sportster modem (having problems hanging up):

The modem would connect to our modem bank just fine, but when he tried to disconnect it would just 'freeze' up. Usually the only way to get out of this situation was a reboot. He found the following tip on a newsgroup, so if you have an older USR Sporster (28.8K that is upgradeable to 33.6K) try changing the S12 register from the default value of 50 to 10. This is done within your Control Panel-Modems-Properties-Connection-Advanced. This page has a section called: Extra Settings. Type in ATS12=10, which will add this command the next time your modem is dialed.

The Un-Official Zoom Info File has the following suggestions to improve your connection to Zoom modems:

If you have a USR Courier or older Sportsers, set S54=80. Newer 92 Mhz Sportsters (with the TI chipset) set S54=16. This disables the 3200 symbol rate.

Check the web site of your modem manufacturer for updated drivers for your operating system. Zoom (our modems of choice) can be found at www.zoomtel.com.

No new modems are being produced with RPI chipsets, they are RPI+. These modems require software drivers to support the modems (rather than having the software built into the modem).
Rockwell has released WinRPI, version 2.26 for Windows 3.1, give it a try at HERE.
WinRPI for WIN95, version 1.15, includes INF file for RPI modems, including v.34 RPI modems. Remember that if you are using Windows 3.1 communication applications within WIN95 you *still* have to change the init strings in the individual applications. Jump HERE for the latest driver.
Rockwell also has a comprehensive FAQ (Frequently Asked Questions) at THIS LINK .