I have been using WIFI calling since I got my S5 and for the most part it has worked well. For the past week i've been getting the "REG99 - Unable to connect" quite frequently. I've tried a few different WIFI networks and get the error on them all. If I reboot a few times it will normally connect, and stay connected for a while. Eventually it comes back, and stays until I go through the reboot cycle again.