*Answer solved through own testing
Originally Posted by trh1341
Wondering the same, about to switch to tmobile.
Well, seems like no one has an answer to it...
Anyway, I went ahead and bought the phone. Since I do a lot of testing and rooting myself, this was the first time working on this type. I'm glad you posted, because I would have just forgotten about it now that I've solved the question I had... However, I figure it'd help in the future in case someone is also wondering the same.
Now, with that all stated... down to the answer.
If you bought an AT&T phone, and do an unlock code to use it on another carrier (such as T-mobile since they use the same towers), the rom you MUST use are the AT&T roms... NOT the T-mobile roms. The problem is, when you buy a AT&T phone, unlock it, and root your device to use another rom, it will not be compatible with T-Mobile roms because of the image files, etc. So yes- you will be losing all the features a T-Mobile rom would have such as wi-fi calling, etc...
However, a way to go about this (and it won't work with ALL the features that comes in a T-Mobile phone/rom), is to install a AT&T rom of your choice without all the crappy AT&T bloatware that you can't even use, and go to the T-Mobile section to look for an APK version of the files. Most of the time, T-Mobile roms will also remove the bloatware, however, some developers are kind enough to provide the bloatware APK files in case users would like to have certain one of them features, such as WiFi Calling.
So, that's how I did it, and it's working good.
PS- One last thing, you may have trouble going on the internet or playstore when you're unlocked to a different carrier. All you need to do is find the APN settings for your carrier. So in this case, I bought an AT&T phone, bought unlock code to use on my carrier T-Mobile, and got the T-Mobile APN settings set up, reboot, and I get all my 4G data, etc. You can get this by calling T-Mobile. Just say, you bought an unlocked phone and can't access data, internet, etc, and was told to set up a APN. Or, you can use google.
Hope I helped everyone else out there!!