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I very much doubt that this is possible, as it constitutes access to the users' local machine - which as you know is a no-no from an untrusted web page.
If it were, you would have all sorts of scam pages using the DOM and JavaScript to have unsuspecting visitors have their modems dial premium rate chat numbers and what not!
I don't have a voice-capable modem and suspect that many people either don't, or don't have the right software installed, or don't know how to use the software. I have one at work, but ironically it's 1) not connected to any phone lines because we have an Ethernet LAN, 2) not connected to a microphone or speakers so I wouldn't be able to talk over it, 3) incapable of negotiating our PBX system so you wouldn't be able to dial out from it anyway.
A connection to a chat system might be a better bet.
you click a button a webpage pops up you enter your phone and then your phone rings, you pick up and then it dials the owner of the buttons phone.
they started this service for the porn industry, so you could talk to the gals.
it has been around for sometime now...
hope that helps
u4ea