I did not find a free open source app for this purpose.. I had very similar needs so I googled for it and found this neat piece of software for the exact purpose mentioned here - making calls over POTS through a computer.. yeah, even I hate getting up to answer the phone when I'm busy with the computer. Besides, I needed a cheaper alternative to buying another instrument for a parallel connection to my other room where the PC is kept.
It is called "IVM Phone answering attendant"
I'm a new member so I'm not able to post URLs so just google for the above phrase and you will find it.
though, here's a try.. nch.com.au/ivm/index.html
There is a trial version which works for 4 days.. I found it nice and it worked with my Voice modem.. please use it and tell me i it worked for you as well.
Enjoy!
