If you have encountered an error that only happens sometimes, such as: hearing an echo or ringing, or the app is unable to connect to a call, you can try force-quitting the app and calling again. This is, in most cases, a random occurrence caused by any number of factors including internet connection, device memory, and microphone feedback, to name a few.
If you're on an Apple device, you can force quit apps following this guideline: https://support.apple.com/en-us/HT201330
If you're on an Android device, you can force quit apps following this guideline: https://www.wikihow.tech/Force-Quit-Apps-on-Android
If you’re on a computer or laptop, try exiting out of Caribu completely and starting it back up. Go to https://app.caribu.com/
Try to limit the number of tabs within your browser you have open, as that can slow down your connection speeds.