We need to find the number of calls in ACD queue. We want to use Snapshot device of the VDN and obtain the list of calls. However, the call fails with error InvalidDeviceID. is there any other possible way

We are also monitoring the ACD split using CallsViaDevice

Calls are not in queue AT the VDN, they are queued AT a skill (hunt group). Snapshot device only works with a station. I believe that "query skill" returns the total number of calls queued at that skill. To know the actual call identifiers for the calls in queue, you would need to have a monitor on the VDN and track QUEUED and DELIVERED and CONNECTION_CLEARED (abandoned) events for calls directed to a skill's queue and keep your own view of what calls are in a QUEUED state. The longer I think it may be more complex than that if the call queues at multiple skills. Some testing with the the DMCC Dashboard or TSAPI Exerciser would provide more clarity on the events received in various call scenarios.