How can I program a VoIP s/w in java

Can anyone tell me if it is possible to write a program in java for sending voice data in the following manner

PC1—>(INTERNET)—> PC2—>(PSTN)—>HOME TELEPHONE.

How do I get the voice data transfered from a pc to a phone? considering i have only a sound card , voice data fax modem, and an internet connection.

do i have to write an entire device driver for the modem to use it to make calls.

Please help.