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).