If an agent, who is logged into Zendesk, takes an incoming call, then the agent will receive a new ticket pushed to their Zendesk view almost immediately. The mechanism used is processed in real-time.
But many other events that happen in calls are added to tickets without real-time processing. For example, when a caller calls your babelforce hotline number and no agent is available, then the caller will be offered a call-back or they can leave a voicemail. Also a caller might hangup before an agent takes the call or without selecting to get a call-back or without leaving a voicemail. All these events cause tickets to be created in Zendesk using non-real time processing. So it could take anything from a few seconds to several minutes for the ticket to appear.
This non-real-time processing is the standard mechanism that is also used for incoming emails and is the most efficient way to handle such events.