I am having the exact same issue with my Google Pixel 2 XL phone. I have had the phone for 6 months and the problem has been going on the entire time. We are also in an area with very poor Verizon coverage, and wifi calling is a necessity. It is very frustrating to try to make or receive a call and find that wifi calling has been turned off. I've contacted Verizon tech support about this issue several times and they have no idea how to solve it other than suggesting factory resets. I had no idea until I found this thread that this is a common problem. Seems like Verizon would come up with a fix for this since it is affecting so many customers. We were using Metro PCS and a Samsung phone before switching to Verizon and the Google Pixel 2 never had a problem with Wifi calling. Due to this issue and some other issues that my Google Pixel was having, I requested and received a warranty replacement phone last week. Wifi calling has already shut off twice on the new phone. As some of the other posters have described, this is a random shut off and when wifi calling is on, it works. My daughter has an iPhone with Verizon and is not having this issue with her wifi calling.