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There is no option to enable it rn. This depends on ur service provider and Xiaomi, both have to enable this option. After that you will be able to do wifi calling. Check list of devices supported for wifi calling on ur service provider's website.
If you use Jio. Open the phone app click on three dots - settings - calling accounts - Jio 4g, If you find wifi-calling under it try to enable it and if it works then your problem is solved. If not then we have no option but wait for them to enable it.