Truecaller's Availability feature makes it possible for you to know if the person you're about to call is busy or not.
For example if you see a red dot next to that number it means that the person is busy, either on a call, in a meeting or has the phone on silent mode. A green dot indicates that the person is free to talk.
You can also see the last time a user was active in the app through the ‘Last Seen‘ status. For more detailed information regarding the Availability feature, please visit the following webpage:
You can always turn the feature off or on in the settings section in Truecaller.
Please note: To be able to see someones availability they must have the app installed and have the feature on.
To disable or enable this feature bring up the menu -> settings -> general.