i have to write a chat application enable to conference calling with voip, but i dont know anything about this technology at all and i couldnt find anything useful. do you know any useful site which help to begin working with it? thanks
Take a look at Asterisk PBX project and Asterisk Java integration project http://asterisk-java.org. Might get some hints and not be forced to reinvent the wheel.
You might look at hosted voice platforms. There's VoiceXML (see http://vxml.org/ for docs and tutorials) and numerous VoiceXML hosts, including my employer, Voxeo ( http://voxeo.com/ ).
There's also Adhearsion, a programming framework that runs on Asterisk http://adhearsion.com/ (it's Ruby, but there's always JRuby if you need to stay in the Java world).
And finally, a number of hosted API services you can use to create voice apps. Tropo ( http://tropo.com/ ), Twilio, ifByPhone ( http://ifbyphone.com/ ), and a few others.
Most of the hosted APIs are designed so you don't need to be a voice developer to understand them. For instance, to create a conference in Java with Tropo, here's the code...
answer();
conference('unique-id');
Using that code, everyone who calls the phone number this code is attached to will be joined into a conference together.