I took the z1s to the t mobile store and they switched phone and sim cards and it still had the same problem. It could receive phone calls and text messages, but not send while using wifi calling. They could not figure it out, so they said that they would wave the restocking fee and let me choose a different phone. I liked this phone so much that I decided to give it a few more days to see if I could fix it.
Well, I did fix it. You should not be having wifi calling problems with this phone and if you do, this is why. Me and my girlfriend purchased these two z1s at the same time, so when the girl at the register scanned the box of my phone, she got them mixed up. She scanned my girlfriends phone as my phone and my phone as hers. So the phone number was not matching the phone scanned to the account. It was backwards. Even when I tried a new phone at t mobile, it was still in reverse, because they were still scanning that phone in under her number instead of the number on my sim card.
So, me and my girlfriend just switched phones. I stuck my sim card in her phone and her sim card in my phone. Everything works great for both phones now.