My call cut automatically and says server is unreachable

You might be using Wi-Fi calling, check in your phone's settings if Wi-Fi calling or "VoIP" is enabled and turn it off, you can also try to make the same call while not connected to Wi-Fi.

this feature lets to route calls over Wi-Fi to improve calls in areas with weak phone signal, but not all phone networks support this fully, and it might not work on some Wi-Fi networks.

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