The service, to be available in 180 countries, will link with existing Yahoo e-mail and instant messaging features. The crowded VoIP sector is getting another major player. Yahoo, which is entering the field with a service that in many ways is cheaper than Web phoning pacesetter Skype, is putting another nail in the coffin of traditional landline telephoning. The Yahoo offering, to be available in 180 countries, will link with existing Yahoo e-mail and instant messaging features, and that appears to be what will distinguish the service from many of its competitors.