For me, this started on 2/12 and when I posted it, a few others chimed in about it, as well. That was error REG91, it later turned into REG99, so your REG 09 sounds completely feasible to be related. It has to do with something that t-mobile has been pushing out. I found out on Friday, that the "master ticket" calling it a global issue (or maybe it was a "global ticket" was written up on February 3, so it was obviously being reported by customers before the wave that happened on the 12th. They push this stuff out in waves, basically using each group as guinea pigs for all subsequent groups, using the current groups reported issues to tweak what they are pushing out and to hopefully push out fixes to the devices that they mess up. Hence, why there are slightly different versions of the error code.
Here is the thread that I started on the 13th and this probably has the most information:
wifi calling...error: REG91 “unable to connect"
And while they were able to get the error resolved, they still haven't been able to correct the quality of the wifi calling that started also as a result of whatever this is that they are pushing out. Here is the thread with details on this:
Earlier this past week, the tech rep that I spoke to was having a live email conversation with the engineering group working on this and they gave an ETA that they would have this fixed by this past Friday. I was promised a call back either way, to let me know, but that didn't happen. I called in again late Friday and they said that it was still open as a "master" or "global" ticket, but that they had no ETA for when it may get fixed. Also, "global" for them does not mean all over the world, but means that the known issue affects several functions of the device (as the terminology was explained to me). I think that this is limited to the devices that have received and keep receiving the update that they are obviously still pushing out, but that the errors and subsequent wifi calling errors happen to smaller numbers of people within those groups. I couldn't get any estimate of how many people that they estimate that this is affecting in various negative ways, so it is tough to get an idea what kind of priority this is getting on their end to get fixed. The master ticket has been open a month already and and this last rep said that he sees master tickets sometimes take several months to fix, but pointed out that the oldest one that he could see open in the system was from October of 2012 and still wasn't fixed.
I am amazed that they have messed up their wifi calling so badly now, recognize it and have already taken a month and still have no ETA to fixing this. WiFi calling is the only feature that separates T-Mobile from every other carrier, so I would think that they would make this a priority to get fixed and to stop pushing this out to phones when they know that there are existing issues. If the ticket was open on the 3rd of February, then they knew about it before pushing it to mine and other's devices on the 12th. And because of all the calling in that they had then and because of these older threads on this site (which I have kept updated), then they were well aware of the issue when they pushed this update out to your phone just a few days ago.