Empathy which is installed by default can handle voice chat. Pidgin will
work with some networks. Gyachi (not in repositories) is made to work with
Yahoo only and reportedly works well. There are no compiled DEBs that I
could find, but there is a PPA. Meebo is a web client that will work

http://www.phrozensmoke.com/projects/pyvoicechat/
ubuntusite.com/.../
How to add Yahoo Voice client in Linux:
www.freeyahootoolsdownload.com/.../get-voice-chat-in-linux-for-yahoo-\
chat.html

This is a bit dated on how to get Gyachi installed in Ubuntu:
www.webupd8.org/.../...ux-ubuntu-gyachi-yahoo.html