There are a couple of things to try:
1) Set up an extension with the same cos. Manually dial the MWI on and MWI off codes to see if there is a delay between when you hit the last digit and when the lamp turns on. If there is a delay, you'll have a digit conflict that you're going to have to find.
2) I don't know the AVT so this "depends". Check all the ports to be sure they are not out of service.
Have you tried just rebooting the AVT?
Ralph