Could they be hang ups that just aren't being dropped by the PBX?

Yes you are right . I also feel that the call maybe hang up by the callee but Still PBX doesn't drop that call yet from the Queue .

also wondering, why are you using Elastix? Hasn't development kinda dried up on that?

Hasn't development kinda dried up on that? can you please give me more details here .

Thank you so much for the reply

I have installed Elastix 4 in a call center . I have 6 agent into my call center . The incoming calls has been routed to the Queue . The Q has 6 agent . Since I have one E1 line which support 30 concurrent call to my Elastix PBX .

The card model is

Digium 1TE235BF Two Span Digital T1, E1, J1 And PRI PCI - Express x1 Card & Hardware Echo Cancellation

The max caller in the Queue setup should be 24. because we have 6 agent and the other 24 should wait in the Queue until we have free available agent . If my Max Caller is 5 in the Queue setup we don't have any silent calls on the agent . When we have 24 or any other number above like 10 , 15 , 20 or 24 in Max Caller then I will receive silent calls on the Call Center agent .

Can you please let me know why do I have such issue . Can we resolve it ? Do you have any solution for it ?

looking forward for your respond

Thank you in advance