Zendesk is a software that logs the IP address of each user. There are other applications that also do the same thing. Other applications we typically see include LiveChat, Olark and Gorgias.
Using IP addresses is typically the easiest way to track down a session. When a customer does not have access to an external application like Zendesk, you are unable to use IP addresses to find sessions.
Not to worry, Noibu has other filtering options to track down a session so if a user calls in about a specific issue, you can still find their session. The easiest way to do this is by searching by date and time.