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.
Some mobile networks, in some countries, will provide you with devices designed to strengthen the network signal, thus allowing you to enjoy the benefits of a full network signal.
Also, there are third party gadgets known as signal boosters, or repeaters, designed to pick up weak signals from the nearest network tower and produce a full powered signal indoors.
Note: Some boosters might use your Internet connection to supplement for a poor network signal. If you live in an urban area and if many people attach to your booster, they will drain your Internet.
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).
The walls and the insulation that is part of them can block signals from the nearby mobile network towers and prevent your device from communicating with it effectively.
Try opening a window and you should be able to increase the strength of the network signal reaching your mobile device.
If you don’t receive the email in the next 2 minutes please check your junk folder, and add [email protected] to your safe sender list.