I have a very similar setup I am also using XPSP3 and A Multitech MT5656ZDX-v Modem. I've tested the modem in windows with voice and audio capabilities working. I have even had Multitech support verify the modem is a voice modem connected and working properly in XP. See the following:
{{Name: Jeff Kline Company:Multi-Tech Systems, Inc. Reason Pending: Requesting More Information
Description: Hi;
No; you do not need to upgrade any firmware. The modem is working fine. I would recommend contacting this company for support on their software. I'm sorry but we do not provide support on user software.
Cheers;
Jeff} Ticket number 5026242}
The modem is seen in the IVM software I'm running version 5.04. It picks up the call but no outgoing message is heard. I cannot hear it on the pc speakers or the call dialed into the software. The call simulator seems to work fine. There are Three choices under the 'Audio' option Intel integrated, Default sound out, and Modem#1 line playback. Currently I have selected the intel option. I look forward to hearing back from anyone who can assist in getting this software to work. I've used older versions in the past and it IS great IF you can get it to work. I wish it had support, such a shame.