You can actually do this a couple ways:
1. get a cheapy regular phone and plug it into an ATA (Analog Telephone Adapter) like one of the Sipura SP1xxx or SP2xxx models or the LinkSys PAP2. An ATA is a box with RJ45 at one end and RJ11 at the other, and a few chips to do things like convert incoming data packets to an analog
signal for the phone, and register with a VOIP server at the other end.

2. Buy an IP Phone from somebody like Zyxel or Sipura.

The ATA route is going to be more compact and better for travel, the IP Phone route is going be more expensive, but fewer pieces involved.

We have a couple of the Sipura 841s and one of the Zyxel WiFi phones at Inveneo, the NGO I work with.

The Sipuras are good as deskphones. They're standard multiline business phones that have an ethernet jack in the back instead of one for a POTS line.

The ZyxelPhone I've got is interesting, but I can't recommend it. It's obviously a first generation product, neat to have around as a WiFi Phone and show off, but it's a little buggy.
They has a new model out, but I haven't used it myself so I can't tell you if it's any better.
Hitachi also has a wireless IP phone, the IP5000

Both of these are pretty expensive, around $350-400 just for the phone.

I personally prefer using a softphone w/ a Bluetooth headset on my Powerbook.

In any case, you're going to need a server at the other end. You can do this via a commercial service like Vonage, Skype, Packet8, et. al. or you can run a VOIP server on a PC somewhere.

Asterisk is a freely available, open source VOIP PBX you can run on most PCs. It runs under Linux, OS X and most modern UNIX systems. I don't know if it runs under 'doze or not.
There is a learning curve in setting it up and getting it running, but it's not terribly difficult for someone with a technical clue.

Also note that if you're going to do the home VOIP server thing and you want to place calls from the Internet to the PSTN, you're going to need a PSTN Adapter card for the PC that's going to be the server.
This does essentially what an ATA does, except that it's a PCI card that fits inside your PC. If you want to be able to both make outgoing and receive incoming calls on the server, you will need to get an Adapter card that supports both FXS and FXO in order to be able to both receive and generate
line ring voltage.

Of all of them, the dedicated ATA w/ a cheapy phone is by far the cheapest solution of all. If you sign up for Vonage or one of the other commercial services, a lot of times they'll just GIVE it to you if you sign up for a year contract. If not, they can be had for around $30-50. The same holds true for the combo wireless router/VOIP gateways from LinkSys you see being flogged at CompUSA/Fry's/Best Buy, et. al.

HTH,
JD