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Use Wi−Fi calling
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.
If there are too many object between you and the network tower you are connected to, the network signal will be weaker. But, the higher the position of the mobile device is, the less there is in the way to block the network signal.
If you live in a two-floor house, try going upstairs to make phone calls and or go out on the balcony (if you live in a house or an apartment/flat).