I know you couldn't add sip extensions to unconditional ring groups until about 4.7/4.8 sw.

I've never tried a sip extension in a ucd group.

What trunks do you have and how many mgi channels vs how many handsets?

You'll likely need 1 mgi port sip phone to make them ring, then 1 additional so if asio phone answers the call it can talk.

Had an issue where we only had 1mgi with a sip phone in a group and it couldn't answer as the mgi was still tired up in making the phone ring.