I have also noticed issues with TMo wifi calling since iOS 10 has been released. In the US, TMo and Sprint have always taken the approach of wifi over cellular, so if wifi calling is enabled and the access point allows it, it is enabled on your phone regardless of cellular signal. AT&T and VZW have taken the approach of cellular over wifi, so wifi calling isn't enabled until cellular signal drops to a level where connectivity might be spotty.
With that said, i've noticed that TMo wifi calling appears to drop connectivity quite frequently. With iOS 9, once it was connected to wifi, it would hold it even when the phone was in standby mode, ready to go when a call would come in. Now, i've noticed that I can pick up my phone every once in a while, and wifi calling will not show up for a minute or so after unlocking the phone, but this doesn't always happen.
FWIW, iOS 10.1 does not appear to correct this issue, and I wonder if this isn't an intentional change to save a little more battery life since the OS doesn't have to hold a wifi connection at all times anymore.