I emailed multitech support and they replied with this:
The modem in question is most likely a voice modem.
To verify this fact use Hyperterminal and give command Ati ENTER.
The command for setting voice mode on this modem is AT+FCLASS=8 and to display its capabilities do AT+FCLASS=? Then ENTER
The GENDER CHANGER should have no effect on the voice capabilities, however, if the GENDER CHANGER is of the wrong type you might not be able to send commands to the modem.
Any thoughts on this? NCH says resolutely if the command ATCLS doesn't fly then it isn't voice... I'm really unimpressed with how far NCH will go to support it's customers. It's ridiculous that their customers have to trip around like this trying to solve problems in implementing their software so much.