(First, be sure that VoIP is turned on in Zencastr's settings menu. If this is turned off, then you will need to use a third-party VoIP solution like Skype or Google Hangouts.)
The VoIP call is not what is actually recorded, but it is needed in order for everyone to hear each other while recording. If someone is disconnected from the VoIP call, they will appear as gray on the recording page -- they will not hear the other participants and others may not hear them.
Check Internet Connection
VoIP disconnections are usually caused by a slow or spotty internet connection. You and your guest(s) can do an internet speed test here: https://www.speedtest.net/. You'll want to see a ping that takes no longer than 150 ms. VoIP will also struggle without upload and download speeds that are consistently at least 1.5-2 Mbps.
Intermittent drops in the internet connection while recording can cause problems at slower internet speeds. This more commonly happens with WiFi. For this reason, hardwired connections are preferred over WiFi connections when possible.
Disable Browser Extensions
Alternatively, if everyone is on a reliable internet connection, the person who is being disconnected might try disabling browser extensions before recording a test call to see if you get a different result. Some security extensions can cause this kind of connectivity trouble, as well, for similar reasons.
Check Network Security Settings
If VoIP disconnections are happening frequently to the same guest, it could be that their network security is interfering with their connection to the Google APIs we use. This would explain the dropping out. This isn't super common, but when we do see it, it's usually from those who are recording from a shared space with tight network security.
There is information here to help in those cases: https://support.zencastr.com/en/articles/2775791-voip-connection-problems-from-my-office-space