Welcome to AndroidCentral Hopefully we can get you sorted.
Making a call depends on a variety of factors. The first being, reception. You need to make sure you have some sort of connection to your network provider. You might need to turn off "Flight Mode" if it's enabled.
The next is having "Airtime" or "Talk Time" available to you. (if you are on a Pre-paid plan). You might need to top up.
The person you are trying to call also needs their phone to be ON and have a connection to their network provider.
Hope this helps!