I swapped out my T-Mobile SIM card yesterday for one of their new Advanced Security SIMs, and WiFi calling suddenly stopped working. I poked around the Internet and found a lot of posts about REG09 and REG99, but not much on REG90 except one misinformed post where someone said that they had to return their phone.
If you swap your SIM and get this message, fear not. Your phone is not broken. Just call up T-Mo (from another phone) and tell them you got that error message. They'll ask you to power down your phone, they'll send an Over-The-Air update, then power your phone on. After that things should work. One thing to note is that newer Android phones really don't "power off" when you select Turn Off. They hibernate. To really power off your phone, you need to hit Restart. This is assuming you have Fast Boot enabled (which is on by default).
The whole process took about 15 minutes, but only because it took a while for me to realize that "Turn Off" isn't really the same as shutting down. Gotta restart for that.
A NOLA native just trying to get by. I live in San Francisco and work as a digital plumber for the joint that runs this thing. (Square/Weebly) Thoughts are mine, not my company's.