**This ALSO fixed my Smart Call Handling issue where it wasn't working, but now is. Follow all steps below!**
I live in the Chicago area, and have T-Mobile.
I use Textra as well since the Z3's stock messenger app clearly CANNOT handle MMS (group/photo text messages) properly. This is what worked for me....................
1) Add a NEW APN (Settings-->More-->Mobile networks-->Access Point Names ((APN))-->push the + sign button) using these settings (and make sure to select it after creating and saving it):
Name: T-Mobile Data 2
Proxy: not set
Port: not set
Username: not set
Password: not set
Server: not set
MMS proxy: not set
MMS port: not set
MMS protocol: WAP 2.0 <---(DOES NOT APPLY TO THE Z3, BUT OTHER PHONE USERS MAY WANNA TRY IT)
Auth Type: not set
APN type: default,mms,supl,admin,hipri
APN proto: IPv4/IPv6
APN roaming proto: IPv4/IPv6
APN enable/disable: enable
2) Disable Stock Messenger app, making sure in "App info" screen (in Settings) that the "Notifications" box is unchecked. (Not sure if unchecking it matters, but I'd do it anyway.) Also, under Stock Messenger's settings, make sure ALL boxes are unchecked as well.
3) In Textra (download this great text SMS/MMS messaging app if you haven't already from Google Play Store), make sure it's your default app, "Use MMS for Group Chats" should be checked, MMS Settings: No Limit, and check the T-Mobile Wifi Calling Fix box too.
4) In Phone settings, under Call Settings, uncheck/disable: VoLTE, Microphone noise suppression, and Speaker voice enhancement. Under Date & time, uncheck Automatic date & time, and Automatic time zone as well. And disable Stamina mode (under Power management) COMPLETELY, including all sub-options.
If you have any questions, let me know.
P.S. For whatever reason, if you're getting an MMS message from someone with an iPhone, and they have "iMessage" turned on, the Z3 will NOT receive the message. So you have to ask people who have iPhones and are sending you MMS messages to kindly turn it off, and then all should be fine.