After several days of playing around (and thus using up any cost savings I had by buying a 16 USD data/fax/voice modem), I got my Trendnet TFM-PCIV92A (B1) modem (which is really just a repackaged Agere modem) working under Win XP SP2. Because I did so much tweaking, I'm not positive what the magic key was, but here is what I think I did:
A) pgodfrin is right. The drivers installed by XP SP2 don't give you voice support.
despite working with the VBRP phone apps that come on the CD, the driver that ships on the CD didn't give TAPI support via UNIMODEM and MODEM.SYS, which is required by IVM; however, there is support commented out in the INF file. So, you can easily enable it as follows:
1. copy the win2k&xp driver files from the CD to a hard drive folder so that you can alter them.
2. edit the AGRSMxp.inf file to make the "pinball modem" a "montblanc" modem by doing the following:
replace: %PINBALL.Modem% = PINBALL_Modem, PCI\VEN_11C1&DEV_0620&SUBSYS_062011C1
with: %MONTBLANC.Modem% = MONTBLANC_SA_Modem, PCI\VEN_11C1&DEV_0620&SUBSYS_062011C1
3. change the montblanc codec to that of the pinball:
replace: HKR,,CodecType,1,2B
with: HKR,,CodecType,1,33
4. goto the Contol Panel/Phone & Modems/ Modem tab, select your modem, click properties. from the drivers tab, update your driver by using the 'install from a specific location', 'i'll pick the location', and 'have disk' option and then pointing it to your newly editted .inf file.
Finally, you may have to reboot for IVM to recognize your modem. you may also have to delete and re-add the modem to IVM. After that, you ought to be up and running.
Like I said above, i've been tweaking and rebooting for several days, so I can't say for sure that this is the magic key, but if not, it should get you close.
Oh, and was noted by someone else, the AT#CLS? still returns "not supported," so that isn't a surefire test of whether you have a voice modem.