Thank you for your reply.
When I open the 'Adjust controls' menu in DCS and use voice attack, it properly highlights the '\' key which is mapped to the comms menu. However, in game, nothing happens.
Interestingly, I created another command to enable the master arm. When I issue this command to voice attack, the master arm switch is enabled. So I think the problem is specific to the communications menu.
As a proof, I mapped the 'HOTAS mic switch down' function to a button on my hotas. Now in game, I can bring up the comms menus with this button. Furthermore, Voice Attack enables me to navigate this menu,. For example, when I say 'F12' since I have created a custom command for F12, I can exit. All other F keys respond to voice attack as well. So the problem is solved.