The short answer is that building a VoIP server is not a simple thing.
It's not impossible for a modem to act as an FXO card (which is what you need to connect your land line to your server) however most modems won't do the job and from what I hear the ones that do work, don't work very well. A better idea would be to use an FXO card like one of these, however this obviously doesn't qualify as "no modifications on hardware": http://www.voipsupply.com/ip-pbx-hardware/pci-cards?s26_no_of_fxo_ports=3486
Check out this link (including the links posted in this thread) and see if your modem is one of the ones that actually would work as an FXO card: http://www.3cx.com/forums/pc-fax-modem-as-fxo-8211.html
You mention that you're not interested in learning Linux or Asterix (I assume you mean Asterisk), but you don't say what operating system you would consider.
Try: http://www.3cx.com/ (Windows) or http://www.pbxnsip.com/mac_pbx (Mac).
Running your own VoIP server is not always advisable due to the *massive* amount of fraud that the industry sees. If you do it, here are a few security tips:
1) Only allow access to trusted IP addresses, if at all possible.
2) Don't use port 5060 - make up a random port.
3) Use strong passwords, with upper case and lower case letters, numbers, and punctuation. Passwords should be 10-20 characters long. (Seriously.)
4) Configure your dial plan so that the server cannot dial to areas that you never need to call, for example, Africa.
5) Use software to regularly monitor your logs and block access to any unauthorized activity. (On Linux systems, we use fail2ban.)
6) If you have a wireless network, use the strongest encryption possible.
7) Never access your VoIP server from a public internet connection.