Voice communication with proprietary protocols like Yahoo! and MSN is a sore point in Linux world. You can try using GYachI for Yahoo! but last I used this application, it was highly unstable and refused to work on lots of occasions. You can give it a try though.
Voice communication with Yahoo! and MSN over SIP is being worked under various projects like OpenWengo and Gizmo Project but they are still under beta stages. Hopefully they will reach the expected stability very soon