[Xperia Z3 Q&A, Help & Troubleshooting] RockStar2005:
Update:

**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 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

APN: fast.t-mobile.com

Proxy: not set

Port: not set

Username: not set

Password: not set

Server: not set

MMSC: http://mms.msg.eng.t-mobile.com/mms/wapenc

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)

MCC: 310

MNC: 260

Auth Type: not set

APN type: default,mms,supl,admin,hipri

APN proto: IPv4/IPv6

APN roaming proto: IPv4/IPv6

APN enable/disable: enable

Bearer: unspecified

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. For WiFi to work, you must also disable anything else you have enabled on this screen (i.e. Microphone noise suppression, Speaker voice enhancement, etc).

5) You can leave STAMINA mode on (under Power management), however...... only enable the "Extended Standby" option and of course Activation should be at 100%/Always on. When Wi-Fi is on, you must also have whitelisted 2 specific apps in STAMINA under the "Apps active in standby" list: The text msg app you're using (i.e. Textra, etc) and I would also whitelist one under "All others" entitled "Phone/Messaging Storage". Again, this is only for when you have Wi-Fi on. Keep the other stuff disabled in here. With regards to STAMINA mode.......

First, put Keep Wi-Fi on during sleep to "Always" (Settings, Wi-Fi, Advanced). Also, make sure everything else on that page is unchecked/disabled. Turn off STAMINA mode. Go into Settings, Apps, All, Extended Standby Mode Controller, Clear Data AND Cache. Then go to your text app's (Textra, etc) "App info" page (on same Apps "All" screen) and clear ONLY the cache, then close out the app itself (3D button to the right of the virtual Home button, swipe the text app off the screen). Reset. When you come back, follow what I said above, including turning STAMINA mode back on ("queue background data" will be enabled. Leave it that way!) Make sure that when you turn STAMINA mode on that the pop-up shows up describing it. If it doesn't at any point, then repeat this paragraph's instructions. What I just said here MAY resolve any lingering MMS issues, and also resolve any issues regarding with STAMINA mode not kicking in too. When I did this my phone spit out a few "missed" incoming MMS messages out all at once. And the battery seems to be last longer too.

It seems that MMS messages will def show up if Wi-Fi is on, but it will take longer to show up if screen is off. Maybe 3-5 minutes longer.

If you have any questions, let me know. I'm still testing this out so I may make more changes. We'll see.

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.

RockStar2005