For me it didn't work after the quest, I assume it is because he is an unbound dremora and after he gave me the sigil stone for me he never left, so now he just walks around the college of winterhold. It's probably a glitch but I always chalked it down to I couldn't summon him again because he is already summoned.
I actually expierienced a wierd glitch that may or may not be related to the spell. Â I cast it, went on with my business in skyrim, and all of a sudden when walking out of the Bee & Barb in riften, a dremora was attacking people! i summoned one and the hostile dremora was still there, Â and when it died, i looted a dremora heart from it, so i guess that using the unbound dremora spell might eventually make an unbound dremora spawn when the screen is done loading. BTW i did this on XBOX
I can’t do it when I have the quest. I think it’s glitching but I don’t know how to make it stop.
Try using Unrelenting Force on Phinis, or try attacking him until he becomes hostile and then immediately yield (might get a fine though so keep a save handy if this fails).