It's either because the battery is dead, the display is broken, or the device has a faulty motherboard and must be replaced.

The phone might be on mute or the speaker is broken, which must be replaced.

Phones calls may not connect if there is a poor signal or the antenna is broken. If the signal is poor, this may be because the cell reception is bad in the area you are in. If the area you are in is supposed to be good, then the antenna may be broken and must be replaced.