It would be great and extremely helpful if you could let us know the make and model of your phone. There are various reasons for your problem to occur and here are two of the more obscure things to check.
For instance, on a Samsung:
Using Safe Mode on a Samsung phone or tablet allows you to force the device to run the essential operating system, and disable most 3rd party applications. It's designed as a diagnostic tool to help solve issues with connectivity, battery life, and more. You might use Safe Mode to test whether or not a 3rd party application is interfering with incoming and outgoing calls.
1: Turn the device off by holding the Power button until the option to Power Off appears on the screen. Tap Power off .
2: Press and Hold the Power and the Volume Down buttons at the same time, to turn the device on.
When you see the Samsung logo appear on the screen, you can let go of the Power button while the device boots up. Make sure to keep holding the Volume Down button.
3: You will now be able to see Safe Mode down the near the bottom left of the screen.
Within Safe Mode you'll be able to test for lots of different issues, navigate around the device to see if you run into any problems.
There are 3 different ways to exit Safe Mode - all of them require the device to restart completely.
1: Hold Volume Down and Power for at least 5 seconds to force the device to restart.
2: Hold the Power button on the right hand side and select Restart on the screen.
3: Swipe down from the top of the screen to access the Notification panel . Select the notification Safe Mode Enabled and tap OK on the prompt to restart the device.
Also, nowadays, there is a system called silent phone...
If you are using an iOS device, please make sure that Silent Phone has access to your device's microphone. You can check this by going to your device's Settings > Privacy > Microphone > Silent Phone and making sure that the switch is turned on (green).
If Silent Phone has access to your microphone but the other party cannot hear you, it could be a problem with your connection or the other person's connection, especially if one of you is behind a firewall.