In your case you need some external application like Asterisk and also external hardware (which is not just a Voice Modem) which can send voice traffic using standard telephone signaling (like Ground Start or Loop Start)
Modem's cant do any of the above activity, however I have a device which can take one line input (std telephone line from mtnl) and shows that as telephone FXO line to Asterisk.
Given that the same modem can be used for voice calls under Windows (E.g. with Windows phone dialer), it should be possible to do the same with Linux.
Blurrb on Windows dialer :
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-...
External hardware (Or better modem) helps. With that, you get multiple PSTN ports and can detect reversal signal for metering or automatic dialing.
But basic voice calling should be possible with this modem itself.
-Shamit