"Session Initiation Protocol (SIP), used for Voice over IP telephony."
VoIP can use SIP, but equally h323 and several others.
SIP is also used (and was orginally designed for), webcasts, IM and many other things.
Also just because 2 devices use SIP, doesn't mean they can talk to each other, as their real time codecs can be completly incompatible.
So not on do you need an open connection (such as SIP) you ideally need an open codec as well.
No here is the crunch, every SIP phone can (should) support g711, but you'll find only some support g729, a licensed codec, the same could happen with Cisco.
To be honest, our systems support about 50 codecs, but we stick with G711, just so it's bloody easier (and sounds better)