Yahoo! Voice
Yahoo Voice was a Voice over IP PC-PC, PC-Phone and Phone-to-PC service, provided by Yahoo via its Yahoo Messenger instant messaging application. Yahoo Voice uses the Session Initiation Protocol (SIP), GIPS codec and the Dialpad engine for voice transport.  Since 2008 the service has been provided by JAJAH, an outsourced VoIP provider. During July 2012, Yahoo voice has been compromised and 453,491 email address and their passwords were hacked. This major devastation was achieved via SQL injection. A 17 MB text file containing the stolen passwords was released by a group of hackers called D33DS company. In relation with this issue, Yahoo advised users to change their passwords as soon as possible.
In Yahoo Messenger Blog, they announced the end of Yahoo Voice on January 30, 2013.

Yahoo Phone Out

PC-to-Phone calls to landlines and mobile phones with the Yahoo Phone Out service (charges apply). Conference calls from PC to Multiple (PSTN) phones or mobiles is currently not supported.

The limitations imposed by packet switched networks, specifically referring to the packet scheduling service (be it over  cable, over the air, etc.), introduce a problematic operation since end users do not always use networks that provide real time packet scheduling. This means no priority and significant delays in transmission of data, which causes delays or "holes" in a conversation. The vast majority of users have best effort packet scheduling services for all their data transmission needs and therefore when user demand is high, or the over the air conditions (radio signal strength, interference ratio and BER) are poor, the conversation appears delayed, cut, bad quality, etc. This behavior is not a VoIP specific problem, but business specific or access technology specific (certain technical aspects must be met for non-delayed data transmission), therefore the same problems are found in any type of VoIP solution.

Yahoo Phone In

Phone-to-PC calls on computer can be received from landline or mobile phones. With the Yahoo Phone In service, one can choose a phone number in three countries (US, UK and France).

Unsupported Functionality 

 Voice call encryption
 PC-Phone (Multiple) Conference calls

See also
 Yahoo Messenger
 VoIP
 Comparison of VoIP software

References

External links
 Yahoo Voice

Teleconferencing
VoIP services
VoIP software
Voice