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Answers: 

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No....The protocol contained by most cases are different. This phone is a SIP (session initiated protocol) Yahoo phone and Skype are totally different. I hope that helps. No, it will not.
Yahoo Phone Out and SkypeOut are not SIP based services.
However, it does support instant message manual chat with Yahoo Messenger, ICQ, AOL, Google Chat using its Jabber client.

As far as VoIP voice call, it will allow you to register with up to 4 different "SIP-based" VoIP services that support BYOD (Bring Your Own SIP Device).
Two SIP-based VoIP services that I individually prefer and subscribe to are InPhonex and CallCentric. (links through my profile).
I also use Les.net, Voip.ms, Ideasip, and SIPphone for making abundant of my SIP-based VoIP calls.
Any of these SIP VoIP services can be configured into the Siemens S450 IP phone.

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