Have you tried calling without Wi-Fi on?
If you had enabled WiFi calling feature on your device when connected to Wi-Fi Mobile device will route your calls and text over Wi-Fi instead of cellular network. This feature is good if you are in poor cellular area.
Reading from your description it looks like calls through Wi-Fi has issues. May be disconnect from Wi-Fi and try the call?
It could be possible that the other person also have network issues. You need to perform these tests considering these different scenarios in mind before finding the problem.
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