saravanapandian,
Why would you ask that question and not just look at the SIP message to see what is supported or not.
Personally, I wouldn't think that REFER would be used by the MiVO-250 at all. I could possibly see it being used between SIP devices on the MiVO-250, but not the MiVO-250 and a Carrier. Actually I don't even see it being used between SIP devices on the MiVO-250 because an Announced Transfer would no longer be an option.
I would say even if a User was talking to an outside person and then for some reason wanted to transfer [REFER] the external user to another outside number the MiVO-250 would create a separate call to allow the initiating person to Announce/Attend the transfer instead of telling the Carrier to do it.
Thanks,
TE