i dont know about this much any way seeing and experiencing that there are not many people to help relating to yahoo account....
i think i can try to help
first thing u can do is
use the help of the yahoo website it self ,they do have huge help ,even for that voice chat ......
next thing i also didnt know this site ever existed....before today
http://www.yahoo-help.com/voice.html
any way try if it helps.....
and from what i can understand ,you need to be invited or you need to invite other for conference ...may be you are trying to enter in to un athorised conference.....
may be you can try things
in yahoo chat first ...
may be some body other may have better idea...