Ah! VoIP. There is a bit of a history about discussing VoIP on flying forums! (Those who know what I am talking about know what I am talking about!)
There are essentially two models for VoIP (Voice over IP).
The simple model requires that both parties use VoIP. The simplest and commonest way of doing this is to use NetMeeting, which is built into Windows. You create a computer connection first, then your headphones are connected to the microphone of the other party, and vice versa.
Next up from this you use a VoIP switch (there are many available) to switch between VoIP telephones (which might be hardware or software) using packet switching.
However, if you need to switch between VoIP and the PSTN you need to go via an interface device. These cost around £150.
There are also ASP services which offer this service, but I don't understand their business model, as they seem to offer free phone calls for no revenue.
Timothy