Checking out the Cisco mpp api and am able to run all other commands except ones that involve a dialpad. Im using the correct call because when I enter a button besides the dialpad it is reflected on the phone such as.
Just guessing, but have you tried other values for "Action", i.e. 0? The docs don't seem to detail what that parameter is precisely, but I'm assuming its something like 0=press+release, 1=press, 2=release...perhaps only a 'press' without a release is happening?