An abandoned call is defined as one where the caller hangs up for whatever reason during the processing of the call (i.e., before the agent answers). Traditionally in many TDM systems, a timer is implemented (or built-in) to allow for a reasonable amount of time to elapse and therefore weed-out these kinds of calls. The timer itself is often quite short depending on the kinds of steps the call goes through during processing. The thought is to remove these short, unproductive calls from the overall statistics of the system. In many cases, there are reports that capture how many of these calls were presented to the system and at what point (in seconds or minutes) the call was terminated. This helps to determine that reasonable timer noted above.
A dequeued call is one that entered the system and stayed connected long enough to be processed by a step or instruction that removes it from one targeted resource/csq and requeues it to a new resource/csq. It is a good thing on many levels. It usually involves "officially" removing statistics from one area and applying them to another which accurately reflects how calls are being handled. It's also a good form of customer service as it provides a level of sensitivity to the call waiting "too long" and attempts to reroute where the call might have a better chance at being answered.
So, two very different things. One is a voluntary action on the part of the caller. The other is a system processing function. Hope that helps.
