I know what both options are. WiFi Calling is supposed to use WiFi to make phone calls, if it is connected, rather than cellular. This makes for fewer dropped calls if in a spotty cellular coverage area, assuming the WiFi signal is good. That parts works very well. WiFi Assist is supposed to allow the cellular connection to step in if your WiFi signal becomes weak or wonky. WiFi Assist was turned ON by default when I upgraded to iOS 9. When my WiFi modem, and therefore WiFi connectivity to internet, tanked a few days ago the iPhone 6+ failed to switch over to cellular in those apps that were set up to use it when WiFi is unavailable. It took a few restarts before Safari and other apps would use the cellular connection to connect. That should have been automatic, and in the past always was automatic. When you walk away from the WiFi area and lose WiFi connection, then the iPhone is supposed to automatically start using cellular to connect for any app that has been set up to do so. That failed. I googled the issue and several websites suggested turning off the WiFi Assist option claiming it might be interfering. Not sure how or if it did interfere, but I turned it OFF. Previously I had no clue that such an option even existed. It really didn't seem to help the issue though. As stated in an earlier post, iOS 9 was having issues with establishing a hotspot from my iPhone 6+ to my iPad and MacBook Pro via WiFi, BT, and USB. It just wouldn't connect. So there are/were some WiFi issues in iOS 9. iOS 9.0.2 has solved my hotspot issues. But just perhaps the WiFi Assist option is somehow messing up the automatic switchover from WiFi to cellular for app connections to the internet.