Weak network signal (mobile device) (I don't live in the USA)

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.

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.