I have no issue with the phone freezing. On my phone, when I am in the process of making a call there will be the wifi calling icon indicating that you are in fact making a wifi call. I have not noticed an icon that will indicate that you will be making a wifi call prior to actually beginning the process of calling someone
On my S6 when on wifi my 4G icon is always grayed out. If I turn off wifi or leave the wifi area it goes right back to 4G. there is no other indication or icon indicating anything. This to me says I'm defaulted to wifi for calling at all times when wifi is on. It did this as soon as I downloaded the update and I have had no issues with it at all. Battery life has improved also.
Please clear your cache if you haven't. May help