What do you mean by "native" SIP?
I've Googled and all I've found is people trying to define it.
SIP isn't, unfortunately, a standard. It's a guideline. It's not black and white as to how it must work. Any SIP phone will work but depending on how the "standard" has been implemented by the phone manufacturer some features may not work. This is the same for any SIP device on any SIP platform.