Here is something you can try:
You stated; "Modem answers incoming call but caller hears nothing." However, you didn't say if you heard the OGM through the PCs speakers - if that happened.
If that's the case you can try to change the default audio out in IVM. Settings/Audio tab : Play Device and Volume
OR/AND
[Windows XP] Control Panel/Sounds and Audio Devices/Audio tab
(I don't have a voice modem hooked up to any Vista PC - so I don't know the path to work with this.)
I had this issue: OGM was playing out through speakers and not through the phone. Followed the IVM help/online help/forum and couldn't tell you exactly what I did that worked. Seems I ended up where I started.
Final settings:
IVM is set to use the modem as the Play Device in Settings/Audio tab
WinXP has sound card in selected in Control Panel/Sounds...
One thing I think was needed. After an IVM and/or OS change I think I needed to reboot for the settings to 'take' as I didn't get what I expected with just the changes made.
Remember, one change one test - takes longer but easier to diagnose.
Good luck.
Kevin