[Xperia Z3 Q&A, Help & Troubleshooting] drummerjames:
You can read all the long drawn out situation, or scroll to the Abridged version down below.

So I've heard of people and developers modding apps, giving the modded versions added features. In fact I have the modded Spotify Apk that allows me to skip any song in my playlist, but moreover from that, there is a serious issue. I don't know if anyone but Xperia Z3 owners on T-Mobile are affected from this, but the stock Sony messaging app on the Xperia Z3 has no MMS/Group Messaging feature. So even if I make a new messaging thread with more than one recipient, after I send the message, the replies come back as if the person sending it, only sent it to me, not the "group thread." So, what? You might be thinking to tell me to "just download a new app that supports MMS and Group Messaging like Textra," but the answer is not as simple as that.

So why is this so important that I'm asking the best developers around the world to help me? Well, T-Mobile is not known for its best signal that covers the entire continent. If fact they're kind of known for the opposite, so much so, that they've implemented "T-Mobile Wi-Fi Calling," to help burden the pain of spotty coverage. So practically all "T-Mobile Wi-Fi Calling" does is it allows one to send SMS/MMS(group messaging and picture)/calls over Wi-Fi. Pretty nifty, right? Well, yeah, if only it allowed third party apps like Hangouts, Textra, chompSMS, etc. to tap into it, but unfortunately, T-Mobile locked the use of "T-Mobile Wi-Fi Calling" only to the stock Sony pre-installed messaging app: Messaging, the app that can't handle a Multi-Recipient MMS group conversation. ("T-Mobile Wi-Fi Calling" is locked to all stock messaging apps, on any T-mobile phone) This problem doesn't just affect the Xperia Z3, it predates all the way back to the original Sony Xperia Z, meaning that all current Sony phone holders are affected by this: Z, ZR, Z1, Z1s, Z1c, Z2, Z3c, and Z3. That is why I'm asking you, the best developers to try and mod the stock Sony Xperia Messaging App, not just to help me, but help a lot of disgruntled Sony phone owners. If the problem has persisted for 8 different phones, I am hardly confident that Sony will fix this in their Lollipop update with the T-Mobile tweaks. All you (the developers/modders) would have to do is mod the stock Sony Xperia Z3 messaging app to have MMS/group messaging handing, rendering it actually usable and usefull. The only other way to resolve this issue is through legalities and laws of app developers getting permission to use and tap into the T-Mobile Wi-Fi Calling, which in all honesty doesn't seem very secure for T-Mobile's data security.

The Sony Xperia Z3 is a great phone, top tier, just with this one annoying quirk. So much so. that people are returning their brand new, perfectly good phones (google "xperia z3 wifi calling mms"). I could just hope that Sony and T-Mobile would finally listen to the thousands complaining, but the chance of that happening is close to zero. In fact the stock Sony Messaging App is actually pretty great and I wish other apps like Textra would implement some of its features (like the ability to set your own custom background on the individual conversation thread), but the likewise could be said in vice versa.

My personal situation (irrelevant to the problem): I don't know if what I typed makes sense (there's a lot of different things to process, and I might of left some details out). I'm just a frustrated Xperia Z3 owner, with the programming experience of copying code from the internet and putting it in the arduino compiler, so I'm way over my head with trying to reprogram/modifying an app. I've searched for months trying to find and decent alternative, but without T-Mobile Wi-Fi Calling I get all my messages an hour to a day late, because of the spotty coverage of the area I live in. I mean, I have 2 bars of 2G all the time, it's just I get 10 seconds of 4G every 15 min, contributing to the delay of messages. The delay (if I even get the message entirely) has messed up some of my friendships, because they think I'm ignoring them. I brag about my phone being so superior to iPhones, and it's always a little embarrassing and humiliating that a phone as good as the Z3 has this one crippling flaw. It's starting to get annoying and I might honestly just switch to an iPhone next because of how well iMessaging works, buts that only if the top of the top developers fail this desperate attempt.

ABRIDGED VERSION: Either I use the stock Sony messaging app that allows MMS over Wi-Fi but no group messaging, or a third party app that allows MMS group messaging but test chance to whether or not I get the message on time, delayed an hour, or not at all. Also the third party app could not utilize T-Mobile Wi-Fi Calling, something many T-Mobile users rely on.

PLAN OF ACTION (for people with the knowhow to fix it [so not me]): Fix the stock Sony Xperia Z3 messaging app by modding it to be able to handle MMS/group messaging. I don't know how, but hopefully some developer/wizard could do some magic that Sony Developers couldn't do with their coconuts they call computers. So all in all is this even possible?