I think Lieberman has certainly been playing it as hard as he can, past the point of silliness, but I have to say that I've not seen anything in any reports that suggest that he deliberately didn't take a call from Gore. All the reports indicate that Gore and Dean didn't want it to leak before the announcement, thus deflating the impact -- which is a reasonable motive -- and that that's what led to Gore not otherwise making prior contact with Lieberman (more doubtful -- that a call just minutes before the announcement couldn't have been set up by a deft staff seems at least questionable).
This is the best evidence I've seen for my speculation. Also Ryan Lizza's TNR piece arguing (persuasively) that Gore's endorsement of Dean really wasn't a surprise.