Hi Alwyn,
When you have started a call, tap on "Keypad" at the bottom of the iPhone. One of the options is "Speaker". Tap on it to activate. (You cannot access "Speaker" until you make the call).
Having got that out of the way - whether on "Speaker" or not, you and the recipient should hear each other perfectly.
The "Speaker" only affects you, allowing you to place the iPhone on a desk or hold it away from your ear. If you bring the iPhone up to your ear, you should still hear it normally.
Now, there is clearly something wrong if the recipient can only hear you if the "Speaker" is on. It should have zero effect as regards the recipient.
What to do? The usual to start with. Power Off the iPhone. If no better, Force Shut Down of the iPhone.
Ian