I live in a house with thick stone walls where I can neither receive nor send mobile calls when I'm inside. Sometimes, when I'm inside, my iPhone switches automatically—without my doing anything—to 'wifi calls'. This can go on for up to a week, switching automatically between mobile calls and wifi calls as I go in and out of the house. I find this very useful as it means I can always send and receive calls.
However, for other periods of up to a week, it switches when I'm in the house to 'no service' when it can't get a mobile signal. How can I make it switch automatically between the two modes all the time?