If you are trying to join a call via internet from the app on your iPhone or iPad and no one can hear you, then it is likely a simple setting on your device that needs to be changed.
For iOS 11: open the 'Settings' app on your iOS device and go to 'Privacy'. Then select 'Microphone', and you will see a list of all apps that have requested access to your microphone. Simply tap the slider next to the App icon so it turns green to enable access to your microphone. Open up the app again and you should now have audio.
If that doesn't resolve the issue, please do submit a ticket.