Hi necostanzo and thanks for reaching out.
You need to have a trouble ticket filed for that error. I would suggest you Contact Us and give our T-Force team a chance to help. They have the tools needed to run this by our engineering team and have this looked into further. They are available 24/7 so you can send them a message anytime through one of the social media links.
I've also been having this issue with REG99 error - I've found that turning wifi on/off/on does not address the issue (ever).
I have found that rebooting the phone AFTER registering with the inflight gogo server works every time!
Not sure why turning WIFI on/off/on or disabling and re-enabling wifi calling doesn't address the issue... Rebooting "stinks" but works!
Have you tried the age old trick of just re-entering your e911 address for that line and saving it? Even if the info is in there it was always the first "trick" to get the REG99 error to go away.