Simple answer is no.
Detailed answer is that voice/fax modem is not the same as PhoneJack. The Phonejack is essentially a sound card, with the G. protocol drivers built into the hardware. With the analog to digital conversion happening in hardware, it is *much* faster than in software. This reduces the delay due to slow software conversion and dramatically reduces the tiem to get your voice to the other party; less lag.
However, with a full duplex sound card, you can skip the Phone Jack AND modem altogether and just do the VoIP in software, like NetMeeting, etc.
Other interesting note is that the software has to be written to take advantage of the PhoneJack driver capabilities, so PhoneJack with Netmeeting is pretty much the same as with a full duplex sound card. The hardware embedded protocols don’t get used unles you use the Internet Switchboard software from Quicknet, or Net2Phone and their driver.
Hope that helps.