Hello, I'm sorry I don't have a solution, but I'm curious what version of CCM and AC you're using? Reason being, I'm having the exact same issue after an upgrade to CCM 6.0(1) and AC version 6.1(0_a).
Thing with ours is, if they leave it alone for maybe 20 seconds while they do something else and wait for a call, when they come back to it, it just turns to a grey box and sits like that for about 5-10 seconds and it'll finally snap out of it and work just fine.