First, ensure that both your iPhone and MacBook Air are connected to the same Wi-Fi network and are signed in to the same iCloud account. This is essential for the Continuity feature to work properly.
Next, check your iPhone's settings to make sure that "Calls on Other Devices" is enabled. You can find this setting by going to Settings > Phone > Calls on Other Devices, and then toggling on the option for your MacBook Air.
Additionally, ensure that FaceTime is enabled on both your iPhone and MacBook Air. You can do this by going to Settings > FaceTime and toggling on the FaceTime option.
If you've checked these settings and are still experiencing issues, try restarting both your iPhone and MacBook Air. Sometimes, a simple restart can resolve connectivity issues.
If the problem persists, you may want to sign out of iCloud on both devices and then sign back in. This can sometimes reset any connectivity issues related to iCloud syncing.
If none of these steps resolve the issue, there may be a more complex underlying problem. In that case, I recommend reaching out to Official Apple Support for assistance.