I'm noticing some changes in the update from 9.3 to 9.3.1. I own a Verizon 6s, and I own a Verizon 3G range extender. With iOS 9.3, I had lots of problems with handoffs between WiFi calling and LTE, so much so that I turned WiFi calling off and used the range extender in the house. Last night (before the update) I decided to try putting my phone into airplane mode with WiFi turned on so that the phone would be forced to use WiFi calling in my house. It seemed to work great. I was able to place WiFi calls with no handoff issues in airplane mode. I did the 9.3.1 update this morning and I can no longer place calls while in airplane mode with WiFi turned on. I can connect to my WiFi network, but I don't see VZW WiFi anymore.
The other interesting thing is that when using a range extender, I previously had to use LTE for data only. Using it for voice would screw up the connection with the range extender. Since the update, I seem to be able to link up with the range extender and place calls through it even though I have LTE set to voice and data.