So for those that are having issues with VoLte and VoWifi not working on T-mobile carrier, I have a simple fix. I currently have the WW version and my cell plan has them both enabled. At the beginning it was working fine for about a month and then it stopped completely, I couldn't receive or make calls or even send text messages. I talked to ASUS tech support and they couldn't figure it out, all they could tell me was to have the same settings on both sim trays but for me I couldn't click on the 2nd tray because it was grayed out. I even went to the trouble of unlocking the bootloader, getting root access but then I couldn't get past diagnostic privileges because it wasn't seeing my device in the settings. I was pretty much at my wits end. I love the phone but it's pretty much for just gaming, that was my thought. Then I thought about what Asus told me about both sim trays need to have the same settings.... when I first inserted my sim in sim tray A (1), T-mobile sent me settings for volte and vowifi to work but the 2nd sim tray didn't have any settings on it, so I decided to switch my sim to the 2nd tray, t-mobile sent me the settings again....WTF.... it worked. Both sim trays are now linked together with the same settings. It's been working fine for over 2 months, reason why I waited to post this info, I wanted to make sure it wasn't a fluke. I have tested it many times with Mobile data off and doing a call over wifi and it's worked fine. So after all the headaches and couple of weeks of fiddling with phone settings, it was a simple sim card switch that fixed it. Hopefully it can help some and for those U.S. peeps that are thinking of getting the phone, now you know the simple way of fixing it.