I found this fix online, applied it exactly, and it worked. It involves editing the Win 10 registry. I am using a US Robotics 5637 USB voice modem. These instructions are for a 64-bit installation of Win 10. If you are using a 32-bit version, you only need to do steps 1 and 3.
Load REGEDIT, then:
1) Goto [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Drivers32]
Find the last wave… parameter, for example, if "wave" and "wave1" exist, then create 1 new entry wave(+1):
"wave2"="serwvdrv.dll"
2) Goto [HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Windows NT\CurrentVersion\Drivers32]
Add the same as in (1)
"wave2"="serwvdrv.dll"
3) Restart the computer.