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Call the operator (carrier) you are subscribed to
It is possible that the operator (carrier) you are subscribed to experiences technical difficulties (e.g. power outage). Try calling their support team, from another device or via their website, and provide them with all the information they ask you for.
If the network signal is weaker but your Wi-Fi signal is good, try calling your friends via an online service such as Viber, Facebook Messenger, etc.
Also, the Wi-Fi calling feature, which is inbuilt in the mobile device's of today allows your phone calls and text to be sent and arrive over the Wi-Fi network and once you are out of the Wi-Fi network, they will arrive on the cellular network.
In order to enable the Wi-Fi calling feature, you need to consult your carrier (operator) and ask them if this feature is supported.
Maybe the operator (carrier) experiences technical difficulties, or carries out a regular maintenance. Wait for no more than 30 minutes. If you still don't get a signal, report the issue to the operator (in the nearest store).