If you are unable to see or hear your friend, please ask them to check that their webcam and microphone are properly connected to their computer and are not being used by any other open programs. They can also verify their settings to make sure their camera and microphone are working correctly.
If you are unable to hear your friend but their microphone works when they test it, there are a few common reasons why you might not hear your friends when using voice and video chat. Please make sure that your speakers are enabled and the volume is turned on. If you're using external speakers, check that they're properly connected to your computer.
Lastly, this issue can be caused by an issue with your friend's network. Voice and video chat tries to establish a direct connection between the two computers calling each other. If this fails, it will fall back on using a proxy server hosted at Google. It is possible that both methods fail, especially if one person has a strict network firewall that blocks all attempts at sending or / receiving data, thus causing a one way call.