I've seen IMS in reference to WiFi calling, is this what you want to do? On my Pixel 2 XL it's done in the phone/dialer app settings.
3-dot menu > settings > calls > configure IMS calling
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Welcome to Android Central! If you can't make or receive calls, are you certain the phone has a SIM inserted, and that the phone supports that network? Did you get the phone from some other carrier and try to bring it to a different carrier? If so, you have to see if the phone has to be carrier-unlocked first.