I have a prepaid account and had been using wifi calling for years. A few months ago, it stopped working. I use wifi at home because of the weak mobile signal at my house. Have meant to go to the local store to inquire but ended up on this forum after searching and seeing something has changed with other types of phones as well (recent discussions on I-Phones for example with wifi calling breaking)
I have an older Android phone and thought it might be related to that. When I turn on wifi calling, I now get a popup saying "Not Registered". You are currently not registered with the T-Mobile network. Would you like to register to enable Wi-Fi Caling?" I hit register and see things saying "Enabling..." Please wait while connecting to T-Mobile Network. It then fails with a message "W006.15 ISP or T-Mobile Network Error." Not sure what this not registered thing is, I'm able to make mobile calls just fine.
Nothing has changed in my home network or on the phone. This just started happening a few months ago. Note this is a T-Mobile branded phone and NOT unlocked.
Interesting thing is that over Thanksgiving, I was on my mom's network (same scenario happened before and that also never used to happen). My sister's boyfriend recently upgraded his t-mobile account and has a prepaid SIM and a postpaid SIM card as he got a new phone number for the new service. With the postpaid "unlimited" SIM in his phone, he enabled Wifi calling and everything was fine and he was able to make wifi calls. He then turned off the phone, put the prepaid SIM in the EXACT SAME PHONE, tried enabling Wifi Caling and got an error "ER: ER05 "invalid SIM"". His prepaid account is still in good standing and was immediately able to make a mobile phone call. His is also an android t-mobile branded phone with a later android version than mine.
So it apparently is NOT just me. His prepaid account won't let him make WIFI calls nor will my prepaid account either. But he can make it on the exact same phone if it's not a prepaid sim card in it.
What's up with this T-Mobile???