What is the make and model of this phone please?
Also, how long have you been facing this issue?
Have you checked the mute button? There is a mute button to the left side of the iphone (just above the volume buttons). Make sure it is not on. If it is on, just turn it off and see if that solves your issue.
If not, let's try a reset. Press and hold the power button and the home button together until you see an apple logo. Let me know if that resolves your issue. If not, we will try the next steps.