If we call someone and they respond we have the addOnCallEstablishedCallback callback to detect that the call was established.
If they don't pick up, the leave a message robot tells us that the person was not able to respond but onCallEstablishedCallback is also called.
So in both of these cases onCallEstablishedCallback is triggered.

Is there a way to distinguish between a call being established with a real person and a call being established with the voice mail robot ?

there should be a difference you can note in the indication of what party answered the call to indicate the called party was not the extension that answered the call (triggered teh established event). The system does not know what is behind any extension (recorded voice, or human). There is a set of services called answering machine detection that is available to outbound dialers under an Advanced TSAPI license using the makePredictiveCall() services if the call was placed using Application Enablement Services CTI services, but I doubt that is your use case.

There's nothing specific indicating that it was forwarded to voicemail, but when it is forwarded to voicemail the call.getRemoteAddress() and call.getRemoteDisplayName() should change. The call.addOnCallRemoteAddressChangedCallback() should fire at that point. Based on local knowledge of the configured voicemail address or display name you might be able to make an assumption.