I've been using Altigen phone systems for a few years. It's not a "modem" based system, but it is PC-based. NT or Windows 2000 server-driven box. It can handle individual phone lines or PRI/T1 lines. You just buy the software and the type of card(s) you need depdending on the line type you're using.
Altigen systems are full scale phone systems used by a lot of major companies, but they have a lot of cool features like the ability to do call center grouping, e-mail voice messages as attachments, etc. They're not "cheap" - I've seen them around $7,000 on eBay but that included the system, cards, software, everything. But they're very scaleable - one system can be upgraded from a one or two line system to handle a 1,000 seat call center just by buying a few cards & software upgrade.
If you're looking for an effective but less expensive solution, one thing to consider is an outsourced unified messaging service like Webley (now CommuniKate). They give you a toll free number which you can forward to your office/home/cell/whatever, screen your calls for you, set up schedules when you are willing to take calls at specific numbers, etc. I have one of those for personal use which calls my home and cell at the same time - no one (not even family) has my home or cell phone number, they all have my CommuniKate number.
Hope this helps!