WiFi Call is a Carrier Specific feature

Wi-Fi calling may frequently disconnect if your cellular service provider and your Internet Service Provider (ISP) have compatibility or peering issues.

When you make a call over Wi-Fi, your mobile carrier relies on the internet connection provided by your ISP to route the call. If there are delays, packet loss, or conflicts between how your carrier and ISP handle traffic—especially if they don’t maintain a strong or cooperative network relationship—you may experience dropped calls, poor audio quality, or connection failures. This is more common with smaller or regional ISPs, or if certain ports or services are throttled or blocked.

WiFi Call is a carrier-specific feature. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you. --> Make a call with Wi-Fi Calling - Apple Support (IN)