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I was under the impression that SIP is a protocol for the initiation of communication between applications, and that the protocol VoIP is for the transmission over an established pair of IP addresses for voice. Therefore the term SIP phone which I encounter does not make sense to me. What are the features which differentiates the two, or if these are alternative names for the same entity?

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closed as off topic by studiohack♦ Oct 14 '11 at 15:41

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There is no difference since SIP is overwhelmingly the protocol of choice for IP telephony. However I guess you could say SIP phones are a (large) subset of VoIP phones. I would generally assume that any VoIP phone is SIP compatible.

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VoIP is a category, it really isn't anything more than a buzz term these days. It is a broad term that includes any and all technologies and protocols that would allow voice over an IP network. SIP is one single protocol in this vast group. Skype has its own proprietary protocol, and is still VoIP. VoIP would also include digital handsets, digital to analog converters, enterprise SBCs, PSTN gateways, and a whole lot more, so there is a huge difference between the two terms. – MaQleod Oct 14 '11 at 16:53
Also, if you're interested, here is the SIP RFC: ietf.org/rfc/rfc3261.txt and a good article to understand SIP: en.wikipedia.org/wiki/Session_Initiation_Protocol and here is a definition of the larger VoIP umbrella: en.wikipedia.org/wiki/Voice_over_Internet_Protocol – MaQleod Oct 14 '11 at 16:58
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Actually VoIP is not a protocol but an umbrella term to cover voice communication over IP. So SIP is considered to be VoIP, and so is Skype.

So it's called a SIP phone since it's using the SIP protocol but wouldn't work with any other VoIP protocol.

Perhaps you were referring to the H.323 protocol? If so, the differences and similarities between H.323 and SIP is covered in Packetizer's excellent article.

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