Buy a mobile device with a wider signal−detection span
If you live in an area where the network reception is weaker, you will need a stronger mobile device. Go to a store and purchase a mobile device which is able to get the maximum of the network you are subscribed to.
It's said that a 2G signal is stronger than 3G, so if the network signal is weaker, try switching to 2G. 3G and 4G are designed to deliver high speed Internet to smartphones, however, at a shorter distance between the network tower and the mobile device.
Note: Please bear in mind that a 2G Internet speed is considerably slower than the one of 3G.
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 your mobile device fails to receive proper signal in a particular area, try forward your calls to another number that works fine in the same environment, provided that your operator supports call forwarding.
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).