Hi Mark,
the Plantronics Hub client SDK is designed to support exactly the kind of situation you just cited. The SDK acts as an interface between your app and the Plantronics device. On the one hand, the SDK allows an app to send commands to devices to direct the behavior of the device, on the other hand, the SDK forwards events from devices (such as button clicks) to the app.
I recommend that you download the latest SDK - spokes 3.5, and install on your development PC. Read the SDK documents and review the sample ccode: SkypeSample installed with the SDK. There are 3 different but functionally identical interfaces to select from: native C++ DLL, COM objects or REST depending on your favorite development language and environment.
What you need to do in your case is receive the answer call event fromt the sdk, then it is up to you to take any action such as sending a CR to your app. See code snippet below:
boolSkypeThread::internalOnCallStateChanged(longid, eCallStatecallstate)
{
if(callstate == CALL_STATE_ACCEPT_CALL)
{
.....
send Return to app
} elseif (callstate == CALL_STATE_TERMINATE_CALL)
{ .....}
Have fun with the SDK.
Wei Chu