Actually there are several:
-hangup
This hangs-up the current call
-answer
Answer an incoming phone call.
-hold
Puts the current call on hold.
-offhold
Takes the on-hold call off hold. If there are multiple lines on hold, calling this multiple times will cycle through all on-hold calls.
-dtmf [key]
This sends a DTMF tone down the line during a phone call. For the "[key]" parameter, you can use any numbers from 0 to 9 inclusive, as well as * and #.
-hide
Hides the window of Express Talk.
-show
Reveals the Express Talk Window
-logon
This starts Express Talk (if it is not open already) and minimizes the program to the system tray.
-exit
This closes Express Talk (complete exit).