does it still work after a reboot?
the first boot after updates wifi calling usually works.
i started typing out an explanation, but it would be a lot easier/better to just copy/paste from the source
"For those that are interested here is some technical details about the bug. The Lumia 640 shipped with Windows 8.1. It also included a special driver made by a third party that handled the IMS protocol and the communication with the mobile operators ePDG server. That is the server that wifi calling connects to that lets you communicate on their network. This driver starts when the phone boots. When upgrading to Windows 10 the exact same driver starts in the exact same way. However Windows 10 services were modified to boot in a different order in order to increase boot time. The driver was now trying to start before one of the dependent services was initialized. The driver failed an API call to initialized IPSEC which is the security layer used in wifi calling. The driver doesn't retry or attempt to reload later. So it can never initialize Wifi Calling."
the initial boot after an update is so slow that the driver often times starts properly. hence wifi calling works until next reboot
all the above info is thanks to travis (who is a microsoft engineer who works on the product team that owns wifi calling)
answers.microsoft.com/en-us/insider/forum/insider_wintp-insider_call/wifi-calling-and-ims-connection-build-10166-and/1ea2a4b5-9b2d-46ad-a3ad-e160686766f4?