You probably need to open ports in the OneCare firewall to resolve this issue. I don't know what ports Yahoo Voice uses, you'll need to contact Yahoo.
Open OneCare, click on Change Settings, Firewall/Advanced/Ports and protocols/Add
If you need help with this, please contact OneCare Support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
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