Hi John,
Lets see if I can answer this one.
If you are not already in a conversation, a small dialog box opens at the bottom right of the screen displaying who is calling and gives you the option to Accept (Alt+C) or Decline (Alt+D).
If you are already in conversation, the same options are available as well as the Alt+C and the Alt+D.
Alt+H is also available when you are done to Hangup.
Hope that helps.