Something I frequently find in call center configurations is a failure to set up lead recycling for system result codes (the dispositions that our system gives to calls as opposed to live agents).
If you are dialing in predictive or progressive modes, our system will finalize certain calls in the background (no answers, busy signals, answering machines, etc). So it is a good idea to check the leads recycling policy for these system dispositions.
In the portal, go to "Projects"> "Manage"> "Leads Recycling Policy" and click on the "System" icon at the top. From there, you can configure the different fields in the columns according to how you want to recycle and process the leads.