When you're busy, when you've got such a full schedule and so much left on your plate, that's when it's even more important to communicate.
I was working in a testing team once and I was very surprised to have nothing to do whereas the developers were extremely busy.
All attempts to understand what the product was about were unsuccessful, email, phone, meetings... They had no time for anything, and I understand why: they were under pressure to deliver as soon as possible.
The only thing's that when the release occurred, we had no clue where to start from, the developers were on a well deserved holiday... And the deadline wasn't missed because of development, but because of testing that took far longer than expected!