I'm continuing to have Wifi calling problems with the ZTE Zmax Pro. We've tried the B04, B07, and Jan 2017 B09 firmware updates. We got a new phone under warranty and a new SIM card and still see the same problems. I'm fairly certain it's an ongoing firmware bug.
I used the Automate app to make a program to record when the wifi calling icon changes. About 5-10 times a day, the calling icon either disappears, we get a REG99, a lot of ER081, or sometimes ER04 error (unable to connect). Resetting the wireless settings and factory resets do not help. After leaving and returning home, wifi calling almost never reconnects. It usually gets REG99 then. We have tried updating our E911 address, and it didn't help. Putting in a SIM card from a different phone didn't stop the problem which stuck with the phone. Rebooting instantly fixes the problem (wifi calling connects), although just for a few hours.
We have the T-Mobile Wifi Cellspot router and have excellent coverage and Internet service. I have a different brand T-Mobile phone that never drops wifi calling, and our previous phones had no problems. I've seen the problems on wifi networks other than ours too.
We have very spotty cell coverage in our house, so I don't know if that is a factor in confusing the Wifi calling software.
I am waiting to hear from T-Mobile support engineers. I hope this can be fixed in the next firmware update. We like the phone, but we rely on wifi calling and are missing calls.
MetroPCS users report similar problems on the ZTE forums...
Thanks in advance for any help!