I had the same issue
When I first loaded the mystery ROM. I ended up going on to my sprint account fron a computer and sniffing around until I found the option for wifI calling. I think it was under the accoint preferences and it worked fine. My buddy who is on my framily plan had the issue also, however after trying a few times, I booted into recovery and wiped user data/factory reset and the cache and then it worked with no issues.