I've got an Avaya IP Office Small Office Edition. The one I have supports eight digital extensions and four analog ones. Faxes work, so there's no good reason that dialing a data connection wouldn't work, although I haven't tried it myself.
I paid a bit under $300 for mine, and it included either five or six phones with it. In a much larger house, I'd actually do some structured wiring and use them for phones, but in my house, I'm basically using it as a surrogate to a normal home phone with voicemail.
There are a few other similar types of systems, Panasonic made a few, Nortel's BCM50 is another, and so on. I liked the Avaya because it has RJ45 ports on the front you can pretty easily adapt to RJ11 ports and just use without having to deal with structured wiring, RJ21/Telco 50 connectors, etc.
If you don't really need a phone system, there are definitely cheaper ways to get two computers talking to one-another using modems.